Fix transaction.sql tests in higher isolation levels.

Fix transaction.sql tests in higher isolation levels.

It seems like a useful sanity check to be able to run "installcheck"
against a cluster running with default_transaction_level set to
serializable or repeatable read.  Only one thing currently fails in
those configurations, so let's fix that.

No back-patch for now, because it fails in many other places in some of
the stable branches.  We'd have to go back and fix those too if we
included this configuration in automated testing.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGJUaHeK%3DHLATxF1JOKDjKJVrBKA-zmbPAebOM0Se2FQRg%40mail.gmail.com