tests: add gem_unref_active_buffers

The crucial trick to reproduce the bug is that we need to have
a decent pile of active bos to retire. Because we unref the bo
after having it moved off the active list, our recursion depth
in fdo bug #42180 is limited by the number of active objects that
can retire at the same time.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42180
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/Makefile.am

@@ -52,6 +52,7 @@ TESTS = getversion \
 	gem_ring_sync_loop \
 	gem_pipe_control_store_loop \
 	gem_hangcheck_forcewake \
+	gem_unref_active_buffers \
 	$(NULL)
 
 HANG = \