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 69e7235c; add them now that we
have the recovery test framework in place.