Comment 4 for bug 2051268

Hmm, I think that the shelve/unshelve test is in error. The /tmp directory cannot be expected to persist across reboots of an instance. https://review.opendev.org/c/openstack/tempest/+/884804 changed the timestamp logic to use the /tmp directory, which works for most if not all other usages of this function but will not for the this test case as the instance is started/rebuilt and most linux systems do not expect /tmp to persist across reboots. Systemd has a unit that does /tmp cleaning for scenarios where /tmp is located on a local disk rather than using tmpfs. Some systems use tmpfs which resides in memory only. Ultimately, this should probably have the timestamp written to an alternate location such as $HOME/timestamp rather than /tmp/timestamp.