Fix subtle timing issue in command/expire tests.
Fix subtle timing issue in command/expire tests.

cdebfb09 added relative times to backup.into but a subtle issue was introduced that would cause the tests to fail if the time acquired by cmdExpire() was exactly the same as timeNow used to format backup.info. cmdExpire() was working correctly given the inputs, but the tests did not run predictably.

This was found while running the tests with --no-valgrind --no-coverage which allows them to run a lot faster, thus exposing the timing issue.

pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest,pgmasters/backrest,pgbackrest/pgbackrest