Simpletest should properly report timeout

This is spin-off of #351805: Endless loop in patch stalls testing - SimpleTest needs a time limit: even if Simpletest is protected from end-less loops now, we need to make sure a fail assertion is reported in case the script timeouts. For this we will need to set up some shutdown function sorcery... while playing nice with the batch API.

That's not easy, but could be done.