tests: Evict stale files in the functional download cache after a while

tests: Evict stale files in the functional download cache after a while

The download cache of the functional tests is currently only growing.
But sometimes tests get removed or changed to use different assets,
thus we should clean up the stale old assets after a while when they
are not in use anymore. So add a script that looks at the time stamps
of the assets and removes them if they haven't been touched for more
than half of a year. Since there might also be some assets around that
have been added to the cache before we added the time stamp files,
assume a default time stamp that is close to the creation date of this
patch, so that we don't delete these files too early (so we still have
all assets around in case we have to bisect an issue in the recent past
of QEMU).

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20251014083424.103202-3-thuth@redhat.com>