Currently we are skipping compilation tests ([* shader] blocks) on
missing capabilities. This is not really necessary and it is useful to
test shader compilation even on machines that can't run these tests.
So, this commit moves the checks to the [test] blocks, skipping them
entirely when caps are not met.
To make the effects of enum test_action values more explicit, these
are renamed to TEST_ACTION_SKIP_COMPILATION and
TEST_ACTION_SKIP_EXECUTION.
fb153bf5d4