`much_older` has to be much older than `older`, or the test gets
flakey. See
- test_cleanup_ondisk_files_reclaim_non_data_files,
- test_cleanup_ondisk_files_reclaim_with_data_files, and
- test_cleanup_ondisk_files_reclaim_with_data_files_legacy_durable
for a more standard definition of "much_older".
Closes-Bug: #2017024
Change-Id: I1eaa501827f4475ddc0c20d82cf0a6d4a5e98f75