pgsql: Add more tests for commit_timestamp feature

Add more tests for commit_timestamp feature

These tests verify that 1) WAL replay preserves the stored value,
2) a streaming standby server replays the value obtained from the
master, and 3) the behavior is sensible in the face of repeated
configuration changes.

One annoyance is that tmp_check/ subdir from the TAP tests is clobbered
when the pg_regress test runs in the same subdirectory.  This is
bothersome but not too terrible a problem, since the pg_regress test is
not run anyway if the TAP tests fail (unless "make -k" is used).

I had these tests around since commit 69e7235c93e2; add them now that we
have the recovery test framework in place.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9def031bd2821f35b5f506260d922482648a8bb0

Modified Files
--------------
src/test/modules/commit_ts/Makefile           |  5 +++
src/test/modules/commit_ts/t/001_base.pl      | 29 ++++++++++++++
src/test/modules/commit_ts/t/002_standby.pl   | 55 ++++++++++++++++++++++++++
src/test/modules/commit_ts/t/003_standby_2.pl | 56 +++++++++++++++++++++++++++
4 files changed, 145 insertions(+)