The expected backend message after SIGQUIT changed in commit
7e784d1d, but we missed updating this test case.  Also, experience
shows that we might sometimes get "could not send data to server"
instead of either of the libpq messages the test is looking for.

Per report from Mark Dilger.  Back-patch to v14 where the
backend message changed.

Discussion: https://postgr.es/m/17BD82D7-49AC-40C9-8204-E7ADD30321A0@enterprisedb.com