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/gem_basic.c

@@ -90,5 +90,5 @@ int main(int argc, char **argv)
 	igt_subtest("create-fd-close")
 		test_create_fd_close(fd);
 
-	return 0;
+	igt_exit();
 }