* tests.[ch]: initialize base so we have a tile cache and add
parameter "gboolean use_cpu_accel" so we can test stuff both with
and without using CPU features.
* tests/test-layer-grouping.c: changed accordingly. Call g_thread_init()
so the tile cache works (and to be on the safe side in general).
Enable us to have regression testing by introducing a core testing
framework that uses the GLib JUnit-like test library.
Do this by adding a new subdir app/tests that will contain all our
tests (it contains one trivial test already). Also add app/tests.[ch]
with a new function gimp_init_for_testing() so that test cases can
easily setup the core object system.
