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_nv_api.c

@@ -569,7 +569,7 @@ int main(int argc, char **argv)
 	intel_batch = intel_batchbuffer_alloc(bufmgr, devid);
 
 #define xtest(name) \
-	if (drmtest_run_subtest(#name)) \
+	drmtest_subtest_block(#name) \
 		if (test_##name()) \
 			exit(2);