In the pg_rewind test suite, receive WAL fully before promoting.

In the pg_rewind test suite, receive WAL fully before promoting.

If a transaction never reaches the standby, later tests find unexpected
cluster state.  A "tail-copy: query result matches" test failure has
been the usual symptom.  Among the buildfarm members having run this
test suite, most have exhibited that symptom at least once.  Back-patch
to 9.5, where pg_rewind was introduced.

Michael Paquier, reported by Christoph Berg.