gtestutils: Add g_test_get_path() API

I found myself wanting to know the test that is currently being run,
where e.g. __func__ would be inconvenient to use, because e.g. the place
the string was needed was not in the test case function. Using __func__
also relies on the test function itself containing the whole path, while
loosing the "/" information that is part of the test path.

docs/reference/glib/glib-sections.txt

@@ -3508,6 +3508,7 @@ GTestDataFunc
 g_test_add_data_func
 g_test_add_data_func_full
 g_test_add
+g_test_get_path
 
 GTestFileType
 g_test_build_filename