lib/drmtest: Add drmtest_subtest_block macro

Doesn't do more than an if (drmtest_run_test(name)) right now, but
as soon as we get a bit of infrastructure to handle test failures and
skipping, this will get more interesting.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/prime_self_import.c

@@ -386,7 +386,7 @@ int main(int argc, char **argv)
 	drmtest_subtest_init(argc, argv);
 
 	for (i = 0; i < ARRAY_SIZE(tests); i++) {
-		if (drmtest_run_subtest(tests[i].name))
+		drmtest_subtest_block(tests[i].name)
 			tests[i].fn();
 	}