gtest: Add more path building API

Add a pair of functions for returning strings that don't need to be
freed.  This is a bit of a hack but it will turn the 99% case of using
these functions from:

  gchar *tmp;
  tmp = g_test_build_filename (...);
  fd = open (tmp, ...);
  g_free (tmp);

to:

  fd = open (g_test_get_filename (...), ...);

which is a pretty substantial win.

https://bugzilla.gnome.org/show_bug.cgi?id=549783