Let each test sets GIMP2_DIRECTORY on their own through the help of a
new test utility function gimp_test_utils_set_gimp2_directory() that
is compiled into its own lib which tests are then linked with. Also,
instead of using "/tmp/gimpdir", we use "gimpdir-empty" in the source
dir. This way we get rid of a bunch of annoying warnings when running
the tests and have more control.
Martin Nordholts