test_qthelpplugin: don't crash after running for >= 60 seconds

All 12 qthelpplugin test cases pass on my computer, but occasionally
(when some Qt Help database is updated?) they run for slightly longer
than a minute. Thus an occasional crash on exit.

Such a crash in test_gdbprinters was worked around in the same way in
efff8aff. For some reason the received
signal is now SegFault instead of SIGPIPE.

CCBUG: 425841