Add googletest wrap file

With this, unit tests can be run in a standalone session.

Tested:
Standalone tests work:

```
1/4 test_systemd_parser                   OK              0.01s
2/4 test_systemd_signal                   OK              0.01s
3/4 test_hypervisor_state                 OK              0.01s
4/4 test_scheduled_host_transition        OK              0.01s

Ok:                 4
Expected Fail:      0
Fail:               0
Unexpected Pass:    0
Skipped:            0
Timeout:            0
```

Change-Id: I434320257498a891af5f15835fdee9927e7d5aee
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/meson.build b/meson.build
index ac253b4..d51a86f 100644
--- a/meson.build
+++ b/meson.build
@@ -278,13 +278,28 @@
 # If test coverage of source files within the root directory are wanted,
 # need to define and build the tests from here
 if build_tests.allowed()
-    gtest = dependency(
-        'gtest',
-        main: true,
-        disabler: true,
-        required: build_tests,
-    )
-    gmock = dependency('gmock', disabler: true, required: build_tests)
+    gtest = dependency('gtest', main: true, disabler: true, required: false)
+    gmock = dependency('gmock', disabler: true, required: false)
+
+    if not gtest.found() or not gmock.found()
+        gtest_proj = import('cmake').subproject('googletest', required: false)
+        if gtest_proj.found()
+            gtest = declare_dependency(
+                dependencies: [
+                    dependency('threads'),
+                    gtest_proj.dependency('gtest'),
+                    gtest_proj.dependency('gtest_main'),
+                ],
+            )
+            gmock = gtest_proj.dependency('gmock')
+        else
+            assert(
+                not get_option('tests').enabled(),
+                'Googletest is required if tests are enabled',
+            )
+        endif
+    endif
+
     test(
         'test_systemd_parser',
         executable(
diff --git a/subprojects/googletest.wrap b/subprojects/googletest.wrap
new file mode 100644
index 0000000..56da9ef
--- /dev/null
+++ b/subprojects/googletest.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/google/googletest
+revision = HEAD