Fix timing issue in pg_rewind test.

Fix timing issue in pg_rewind test.

The test inserts a row in primary server, waits until the insertion has
been replicated to a cascaded standby, and checks that it's visible there
by querying the cascaded standby. In order for that to work reliably, the
test needs to wait until the insertion WAL record has been fully replayed.

This should fix the occasional buildfarm failures. Diagnosis by Tom Lane.

Discussion: https://www.postgresql.org/message-id/606796.1605424022@sss.pgh.pa.us