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_concurrent_blit.c

@@ -366,5 +366,5 @@ main(int argc, char **argv)
 	for (i = 0; i < ARRAY_SIZE(access_modes); i++)
 		run_modes(&access_modes[i]);
 
-	return 0;
+	igt_exit();
 }