Fix pg_rewind regression failure after "fast promotion"

Fix pg_rewind regression failure after "fast promotion"

pg_rewind looks at the control file to determine the server's timeline. If
the standby performs a "fast promotion", the timeline ID in the control
file is not updated until the next checkpoint. The startup process requests
a checkpoint immediately after promotion, so this is unlikely to be an
issue in the real world, but the regression suite ran pg_rewind so quickly
after promotion that the checkpoint had not yet completed.

Reported by Stephen Frost