Stabilize timeline switch regression test.

 Stabilize timeline switch regression test.
 
 Commit fef5b47f6b added the regression test to check whether a standby is
 able to follow a primary on a newer timeline when WAL archiving is enabled.
 But the buildfarm member florican reported that this test failed because
 the requested WAL segment was removed and replication failed. This is a
 timing issue. Since neither replication slot is used nor wal_keep_size is set
 in the test, checkpoint could remove the WAL segment that's still necessary
 for replication.
 
 This commit stabilizes the test by setting wal_keep_size.
 
 Back-patch to v13 where the regression test that this commit stabilizes
 was added.
 
 Author: Fujii Masao
 Discussion: https://postgr.es/m/X//PsenxcC50jDzX@paquier.xyz