Eschew "RESET statement_timeout" in tests.

Instead, use transaction abort.  Given an unlucky bout of latency, the
timeout would cancel the RESET itself.  Buildfarm members gharial,
lapwing, mereswine, shearwater, and sungazer witness that.  Back-patch
to 9.1 (all supported versions).  The query_canceled test still could
timeout before entering its subtransaction; for whatever reason, that
has yet to happen on the buildfarm.