tests: use igt_exit() consistently with subtests

This is mostly important to get the SKIP reporting right, but I've
found a few stragglers that wanted to get converted over to the igt
result reporting completely.

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

tests/prime_nv_api.c

@@ -594,5 +594,5 @@ int main(int argc, char **argv)
 	close(intel_fd);
 	close(nouveau_fd);
 
-	return ret;
+	igt_exit();
 }