app: Add GIMP core unit testing framework and a simple first test
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.
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