Allow for yet another crash symptom in 013_crash_restart.pl.

Allow for yet another crash symptom in 013_crash_restart.pl.

Given the right timing, psql could emit "connection to server was lost"
rather than one of the other messages that this test script checked for.
It looks like commit 4247db625 may have made this more likely, but
I don't really believe it was impossible before then.  Rather than
stress about it, just add that spelling as one of the crash-successfully-
detected cases.

Discussion: https://postgr.es/m/19344.1548554028@sss.pgh.pa.us