libtracker-common: improve test_path_evaluate_name environment handling

Under some environments $PWD might not point where we expect, so simply use g_get_current_dir() instead. g_getenv() is documented to return a pointer that may get overwritten by subsequent calls to g_getenv, g_setenv of g_unsetenv. As even after removing the second g_getenv, there's still g_setenv calls, take a copy.