Unit tests: Explicitly check for success from runner.

We will otherwise miss runner failures for any file that does not
contribute to the diagnostics count.
Note: The tool can also fail during the building stage, but in this case
we will get notified by the signal spy timing out.

Change-Id: Ia9aa797d658b1752e3da6e08a652ee55868955ba
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>