This allows tests to skip when required capabilities are not present.
The output damage test for the shadow buffer case needs this.
required_capabilities is added to struct weston_testsuite_quirks which
is libweston public API just because there is no better place currently.
This is a little weird because the code to check it is in compositor,
not libweston.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>