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