Add extension options to control TAP and isolation tests

Hi all,

On a recent thread of pgsql-committers has been discussed the fact that
we lacked a bit of infrastructure to allow extensions to control
isolation and TAP tests:
https://www.postgresql.org/message-id/20180905174527.GA2726@paquier.xyz

Attached is a patch which is the result of the previous thread, where a
couple of variables are added for extension authors:
- ISOLATION, similar to REGRESS for pg_regress, which lists isolation
tests.
- ISOLATION_OPTS, which can be used to pass an option set to
pg_isolation_regress.
- TAP_TESTS, a switch to enable running TAP tests.

This results in a bit of cleanup in the Makefile of modules we ship with
upstream:
- modules/brin
- modules/commit_ts
- modules/test_pg_dump
- contrib/bloom, where the TAP tests were actually not running.
- contrib/oid2name
- contrib/test_decoding, which keeps the same feature set, and is
heavily cleaned up (it missed for example the use of the existing
NO_INSTALLCHECK).
- contrib/vacuumlo

Thoughts?
--
Michael