pgsql: Improve isolation tests infrastructure.

Improve isolation tests infrastructure.

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

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/60f826c5e62446d211e15ae31710562a26afc442

Modified Files
--------------
contrib/test_decoding/.gitignore             |  5 +++--
contrib/test_decoding/Makefile               |  7 +-----
src/Makefile.global.in                       | 33 +++++++++++++++++++++-------
src/test/isolation/.gitignore                |  5 ++---
src/test/isolation/Makefile                  |  8 +++----
src/test/modules/snapshot_too_old/.gitignore |  2 +-
src/test/modules/snapshot_too_old/Makefile   |  6 ++---
src/test/regress/pg_regress.c                |  6 ++++-
8 files changed, 44 insertions(+), 28 deletions(-)