BUG #15594: Unstable tests in contrib/test_decoding/output_iso/

The following bug has been logged on the website:

Bug reference: 15594
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 11.1
Operating system: Ubuntu 18.04
Description:

Some tests in contrib/test_decoding/output_iso/ occasionally fail.
To reproduce it reliable you can do:
# sed 's/ mxact / $(shell printf "mxact %.0s" `seq 1000` ) /' -i
contrib/test_decoding/Makefile
# make check -C contrib/test_decoding/

...
test delayed_startup ... ok
test ondisk_startup ... ok
test concurrent_ddl_dml ... ok
test oldest_xmin ... ok
test snapshot_transfer ... ok
============== shutting down postmaster ==============

==========================
19 of 1005 tests failed.
==========================

The differences that caused some tests to fail can be viewed in the
file ".../contrib/test_decoding/output_iso/regression.diffs". A copy of the
test summary that you see
above is saved in the file
".../contrib/test_decoding/output_iso/regression.out".

The diff file contains:
--- .../contrib/test_decoding/expected/delayed_startup.out 2018-02-08
16:57:42.816744192 +0300
+++ .../contrib/test_decoding/output_iso/results/delayed_startup.out
2019-01-16 17:46:33.138962021 +0300
@@ -12,6 +12,8 @@
step s2start: SELECT data FROM
pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids',
'false');
data

+BEGIN
+COMMIT
step s1b: BEGIN ISOLATION LEVEL SERIALIZABLE;
step s1w: INSERT INTO do_write DEFAULT VALUES;
step s1c: COMMIT;

======================================================================
...

I observe this issue with the tests: mxact, delayed_startup, and
ondisk_startup.

Best regards,
Alexander