Previously if a directory had both isolationtester and plain
regression tests, they couldn't be run in parallel, because they'd
access the same files/directories.  That, so far, only affected
contrib/test_decoding.

Rather than fix that locally in contrib/test_decoding, improve
pg_regress_isolation_[install]check to use separate resources from
plain regression tests.

That requires a minor change in pg_regress, namely that the
--outputdir is created if not already existing, that seems like good
idea anyway.

Use the improved helpers even where previously not used.

Author: Tom Lane and Andres Freund
Discussion: https://postgr.es/m/20170311194831.vm5ikpczq52c2drg@alap3.anarazel.de