pgsql: Revert all new recent changes to add PGXS options for TAP and is

Revert all new recent changes to add PGXS options for TAP and isolation

A set of failures in buildfarm machines are proving that this is not
quite ready yet because of another set of issues:
- MSVC scripts assume that REGRESS_OPTS can only use top_builddir. Some
test suites actually finish by using top_srcdir, like pg_stat_statements
which cause the regression tests to never run.
- Trying to enforce top_builddir does not work either when using VPATH
as this is not recognized properly.
- TAP tests of bloom are unstable on various platforms, causing various
failures.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1d7dd18686d0373289a3bf2b3669edae20875cbc

Modified Files
--------------
contrib/bloom/Makefile | 7 ++--
contrib/oid2name/Makefile | 10 ++++-
contrib/test_decoding/Makefile | 67 ++++++++++++++++++++++--------
contrib/vacuumlo/Makefile | 10 ++++-
doc/src/sgml/extend.sgml | 61 +--------------------------
src/makefiles/pgxs.mk | 32 +-------------
src/test/modules/brin/.gitignore | 2 +-
src/test/modules/brin/Makefile | 25 +++++++++--
src/test/modules/commit_ts/Makefile | 6 ++-
src/test/modules/snapshot_too_old/Makefile | 35 ++++++++++++----
src/test/modules/test_pg_dump/Makefile | 6 ++-
11 files changed, 133 insertions(+), 128 deletions(-)