Add PGXS options to control TAP and isolation tests

The following options are added for extensions:
- TAP_TESTS, to allow an extention to run TAP tests which are the ones
present in t/*.pl.  A subset of tests can always be run with the
existing PROVE_TESTS for developers.
- ISOLATION, to define a list of isolation tests.
- ISOLATION_OPTS, to pass custom options to isolation_tester.

A couple of custom Makefile targets have been accumulated across the
tree to cover the lack of facility in PGXS for a couple of releases when
using those test suites, which are all now replaced with the new flags,
without reducing the test coverage.  This also fixes an issue with
contrib/bloom/, which had a custom target to trigger its TAP tests of
its own not part of the main check runs.

Author: Michael Paquier
Reviewed-by: Adam Berlin, Álvaro Herrera, Tom Lane, Nikolay Shaplov,
Arthur Zakirov
Discussion: https://postgr.es/m/20180906014849.GG2726@paquier.xyz

contrib/bloom/Makefile

@@ -8,6 +8,7 @@ DATA = bloom--1.0.sql
 PGFILEDESC = "bloom access method - signature file based index"
 
 REGRESS = bloom
+TAP_TESTS = 1
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config
@@ -19,6 +20,3 @@ top_builddir = ../..
 include $(top_builddir)/src/Makefile.global
 include $(top_srcdir)/contrib/contrib-global.mk
 endif
-
-wal-check: temp-install
-	$(prove_check)