libweston: add required_capabilities test suite quirk

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>

tests/weston-test-fixture-compositor.c

@@ -42,6 +42,9 @@
 #include "weston.h"
 #include "test-config.h"
 
+static_assert(WET_MAIN_RET_MISSING_CAPS == RESULT_SKIP,
+	      "wet_main() return value for skip is wrong");
+
 struct prog_args {
 	int argc;
 	char **argv;