tests: Test wl_interface_equal

The test suite did not cover wl_interface_equal, so add one test that
specifically tests this method. Add the new test to Makefile.am.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
[Pekka: add assert(&wl_registry_interface != &copy);]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>