pgsql: Reduce cost of test_decoding's new oldest_xmin test

Reduce cost of test_decoding's new oldest_xmin test

Change a whole-database VACUUM into doing just pg_attribute, which is
the portion that verifies what we want it to do.  The original
formulation wastes a lot of CPU time, which leads the test to fail when
runtime exceeds isolationtester timeout when it's super-slow, such as
under CLOBBER_CACHE_ALWAYS.  Per buildfarm member friarbird.

It turns out that the previous shape of the test doesn't always detect
the condition it is supposed to detect (on unpatched reorderbuffer
code): the reason is that there is a good chance of encountering a
xl_running_xacts record (logged every 15 seconds) before the checkpoint
-- and because we advance the xmin when we receive that WAL record, and
we *don't* advance the xmin twice consecutively without receiving a
client message in between, that means the xmin is not advanced enough
for the tuple to be pruned from pg_attribute by VACUUM.  So the test
would spuriously pass.

The reason this test deficiency wasn't detected earlier is that HOT
pruning removes the tuple anyway, even if vacuum leaves it in place, so
the test correctly fails (detecting the coding mistake), but for the
wrong reason.

To fix this mess, run the s0_get_changes step twice before vacuum
instead of once: this seems to cause the xmin to be advanced reliably,
wreaking havoc with more certainty.

Author: Arseny Sher
Discussion: https://postgr.es/m/87h8lkuxoa.fsf@ars-thinkpad

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/aba2184bed64ef3b6f01465df940eb560d5f772d

Modified Files
--------------
contrib/test_decoding/expected/oldest_xmin.out | 7 +++++--
contrib/test_decoding/specs/oldest_xmin.spec   | 9 ++++++---
2 files changed, 11 insertions(+), 5 deletions(-)