lib: Unit test for exit handler

Checks that
- exit handlers are run only once, even when registered multiple
  times.
- run in reverse order
- actually run for all ways a test could exit.

This is prep work to extend exit handlers to also work in a subtest
aware way.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

lib/tests/Makefile.sources

@@ -11,6 +11,7 @@ check_PROGRAMS = \
 	igt_invalid_subtest_name \
 	igt_segfault \
 	igt_assert \
+	igt_exit_handler \
 	$(NULL)
 
 check_SCRIPTS = \