build-unit-test-docker.sh: Install iproute2 for `ip` utility

`ip` is required by phosphor-networkd, which `execl`s out to delete vlan
interfaces.

Without iproute2 installed we get spurious test failures:

    [----------] 3 tests from TestVlanInterface
    [ RUN      ] TestVlanInterface.createVLAN
    [       OK ] TestVlanInterface.createVLAN (4 ms)
    [ RUN      ] TestVlanInterface.deleteVLAN
    [       OK ] TestVlanInterface.deleteVLAN (4 ms)
    [ RUN      ] TestVlanInterface.createMultipleVLAN
    [       OK ] TestVlanInterface.createMultipleVLAN (8 ms)
    [----------] 3 tests from TestVlanInterface (16 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30075 ms total)
    [  PASSED  ] 26 tests.
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.6j14Sy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (9 ms)
    [----------] 3 tests from TestVlanInterface (17 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30076 ms total)
    [  PASSED  ] 25 tests.
    [  FAILED  ] 1 test, listed below:
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     1 FAILED TEST
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.6j14Sy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (10 ms)
    [----------] 3 tests from TestVlanInterface (18 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30078 ms total)
    [  PASSED  ] 25 tests.
    [  FAILED  ] 1 test, listed below:
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     1 FAILED TEST
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.6j14Sy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (11 ms)
    [----------] 3 tests from TestVlanInterface (19 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30078 ms total)
    [  PASSED  ] 25 tests.
    [  FAILED  ] 1 test, listed below:
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     1 FAILED TEST
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.I5nIok]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.deleteVLAN (16 ms)
    [ RUN      ] TestVlanInterface.createMultipleVLAN
    [       OK ] TestVlanInterface.createMultipleVLAN (6 ms)
    [----------] 3 tests from TestVlanInterface (26 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30085 ms total)
    [  PASSED  ] 25 tests.
    [  FAILED  ] 1 test, listed below:
    [  FAILED  ] TestVlanInterface.deleteVLAN

     1 FAILED TEST
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.fhR6Uy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (7 ms)
    [----------] 3 tests from TestVlanInterface (27 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30086 ms total)
    [  PASSED  ] 24 tests.
    [  FAILED  ] 2 tests, listed below:
    [  FAILED  ] TestVlanInterface.deleteVLAN
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     2 FAILED TESTS
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.fhR6Uy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (8 ms)
    [----------] 3 tests from TestVlanInterface (28 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30087 ms total)
    [  PASSED  ] 24 tests.
    [  FAILED  ] 2 tests, listed below:
    [  FAILED  ] TestVlanInterface.deleteVLAN
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     2 FAILED TESTS
    unknown file: Failure
    C++ exception with description "filesystem error: cannot remove all: No
    such file or directory [/tmp/VlanInterface.fhR6Uy]" thrown in
    TearDown().
    [  FAILED  ] TestVlanInterface.createMultipleVLAN (8 ms)
    [----------] 3 tests from TestVlanInterface (28 ms total)

    [----------] Global test environment tear-down
    [==========] 26 tests from 6 test cases ran. (30087 ms total)
    [  PASSED  ] 24 tests.
    [  FAILED  ] 2 tests, listed below:
    [  FAILED  ] TestVlanInterface.deleteVLAN
    [  FAILED  ] TestVlanInterface.createMultipleVLAN

     2 FAILED TESTS
    FAIL test (exit status: 1)

Note that the one test run is displaying multiple instances of the
global test environment tear-down. There should only ever be one per
gtest_main binary.

The `execl` invocation fails after we've forked[1], and we throw an
InternalFailure exception as a consequence. As the process image is
still identical to the parent, we unwind and clean up the test directory
via the TestVlanInterface destructor, then exit. The exit triggers the
parent to break out of its wait-loop for the child, which throws its own
InternalFailure exception. Again we hit the TestVlanInterface
destructor, but as confDir has been cloned across both of the processes
we try to remove a directory that no-longer exists. The test then exits
with the filesystem_error exception.

The successful `fork`s combined with failed `execl`s leads to a cascade
of cleanup failures and the consequent failure of the test suite.

Install `iproute2` so `execl` will succeed.

[1] https://github.com/openbmc/phosphor-networkd/blob/master/util.cpp#L369

Tested: Observed phosphor-networkd pass its test suite using
        test-distro.sh with ubuntu:artful as the base image

Change-Id: Ibbe01926c744a931da14542afc372a69d02bc99b
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index ad7084b..1847b24 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -77,7 +77,8 @@
     git \
     dbus \
     iputils-ping \
-    clang-format-5.0
+    clang-format-5.0 \
+    iproute2
 
 RUN easy_install pip
 RUN pip install inflection