test: Change tmp file names to fix races

With MAKEFLAGS=-jN for N > 1 we hit race conditions in the test suite.
Multiple independent tests use the same absolute path under /tmp for
test case IO. This leads to corruption across the tests manifesting as
intermittent failures. Make the file-names semi-unique.

This solution is poor in that it makes no attempt at ensuring the file
names are absolutely unique, but they are at least unique wither respect
to the other test file names, which is an improvement.

Change-Id: I6b312093a2d9cd52261d02821566b69e874e7770
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/test_watch.cpp b/test/test_watch.cpp
index dac43b3..3ab3e22 100644
--- a/test/test_watch.cpp
+++ b/test/test_watch.cpp
@@ -6,7 +6,7 @@
 #include <fstream>
 #include <experimental/filesystem>
 
-static constexpr auto TRIGGER_FILE = "/tmp/netif_state";
+static constexpr auto TRIGGER_FILE = "/tmp/" __BASE_FILE__ "netif_state";
 
 namespace fs = std::experimental::filesystem;