Support an overall timeout, in addition to testitem_timeout

[nickrobinson251]

Currently we have the ability to "timeout" an individual @testitem, i.e. set a time limit at which we forcefully stop running a test-item and mark it as failed.

But we do not have the ability to set a timeout for the total time of runtests... which could be convenient

e.g. if you want to split a large test suite into multiple test jobs and don't want any test job to take more than 1h, you might want to set timeout=3600 (perhaps in addition to e.g. requiring each test-item completes in <5 minutes by setting testitem_timeout=300)