pg_regress: Promptly detect failed postmaster startup.

Detect it the way pg_ctl's wait_for_postmaster() does.  When pg_regress
spawned a postmaster that failed startup, we were detecting that only
with "pg_regress: postmaster did not respond within 60 seconds".
Back-patch to 9.4 (all supported versions).

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20181231172922.GA199150@gust.leadboat.com