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@gmail.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