Fix and improve TAP tests for pg_hba.conf and regexps

Fix and improve TAP tests for pg_hba.conf and regexps

The new tests have been reporting a warning hidden in the logs, as of
"Odd number of elements in hash assignment" (perlcritic or similar did
not report an issue, actually).  This comes down to a typo in the test
"matching regexp for username" for a double-quoted regexp using commas,
where we passed an extra argument.  The test is intended to pass, but
this was causing the test to fail.  This also pointed out that the
newly-added role "md5,role" lacks an entry in the password file used to
provide the password, so add one.

While on it, make the tests pickier by checking the contents of the logs
generated on successful authentication.

Oversights in 8fea868.