Add a status return value to every test definition. There are no
behavioral changes, previously the RESULT_OK was simply assumed.
The benefit of this is that in the future individual tests can also
return RESULT_SKIP, so that we can keep statistics of skipped tests.
ivi-layout-internal-test.c has the only case where a test function may
return early. That one is set to return RESULT_HARD_ERROR to match the
compositor exit code already there.
Also documentation is updated.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
