The testlib selftest shouldn't rely on matching data after a QFATAL

As explored in QTBUG-73898: when a test program exercises undefined behaviour, as the crashes selftest does, we can't rely on anything about its output. So, at least for the crashes test, matching of actual and expected output needs to be somewhat flexible from the moment the expected output includes a QFATAL; once we've seen (and matched) a QFATAL, or the next expected line is one, the matching needs to accept actual output that stops prematurely.