fix: correct check of the updated state

Commit 8f826964 contain the fix of ostree commit ID parsing.
Before that commit the ID contained additional suffix (".0") and
thus the comparison with `COMMIT_AFTER` always shows a difference
and the test result was __always__ false positive: "pass".

After applying the 8f826964 commit we shed light to other issue --
we saved a wrong ostree commit ID for later usage.
In case if we prepare the "outdated" commit -- we must use it's ID
as "starting" for the test.

Moved the preparation of "outdated" ostree commit into the common
function `prepare_outdated_commit()` and store the proper ostree
commit ID as starting point.

Additional side-effect fixed for OTA upgrades -- before current fix
OTA tests were unstable, sometimes allowing to pass the check even
with wrong ID written as starting point.
This is caused by long queues in LAVA nowadays -- the build system may
prepare and submit the new version while the test is waiting in queue.
In that case the system do the real upgrade to the OS version newer
than we have during the test submit.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>