test: properly catch tests error with no /testok or empty /failed

When editing this function in 7bf20e48, I couldn't
decide whether to initialize ret at the top and only reset it on success, or
whether to assign a value in each branch. In the end I did neither ;( So if the
test finished without creating any of the result files, we would echo a
message, but return "success".

But there was bigger confusion with /failed: some tests create it empty, some
don't. I think we may want to do away pre-creation of /failed completely, and
assume the test failed unless /testok is found. But I'm leaving that for later
rework. For now let's just make sure we report return success only if /testok
or /skipped is found.