Test restartpoints in archive recovery.

 Test restartpoints in archive recovery.
 
 v14 commit 1f95181b44c843729caaa688f74babe9403b5850 and its v13
 equivalent caused timing-dependent failures in archive recovery, at
 restartpoints.  The symptom was "invalid magic number 0000 in log
 segment X, offset 0", "unexpected pageaddr X in log segment Y, offset 0"
 [X < Y], or an assertion failure.  Commit
 3635a0a35aafd3bfa80b7a809bc6e91ccd36606a and predecessors back-patched
 v15 changes to fix that.  This test reproduces the problem
 probabilistically, typically in less than 1000 iterations of the test.
 Hence, buildfarm and CI runs would have surfaced enough failures to get
 attention within a day.
 
 Reported-by: Arun Thirupathi <arunth@google.com>
 Discussion: https://postgr.es/m/20250306193013.36.nmisch@google.com
 Backpatch-through: 13