TAP tests: check for postmaster.pid anyway when "pg_ctl start" fails.

TAP tests: check for postpid anyway when "pg_ctl start" fails.

"pg_ctl start" might start a new postmaster and then return failure
anyway, for example if PGCTLTIMEOUT is exceeded.  If there is a
postmaster there, it's still incumbent on us to shut it down at
script end, so check for the PID file even though we are about
to fail.

This has been broken all along, so back-patch to all supported branches.

Discussion: https://postgr.es/m/647439.1642622744@sss.pgh.pa.us