Make the pg_rewind regression tests more robust on slow systems.

Make the pg_rewind regression tests more robust on slow systems.

There were a couple of hard-coded sleeps in the tests: to wait for standby
to catch up with master, and to wait for promotion with "pg_ctl promote"
to complete. Instead of a fixed, hard-coded sleep, poll the server with a
query once a second. This isn't ideal either, and I wish we had a better
solution for real-world applications too, but this should fix the
immediate problem.

Patch by Michael Paquier, with some editing by me.