* Use add-on config timestamp to determine add-on update age
Instead of using the current timestamp when loading the add-on config,
simply use the add-on config modification timestamp. This way, we can
get a timetsamp even when Supervisor got restarted. It also simplifies
the code a bit.
* Fix pytest
* Patch stat() instead of modifing fixture files
* Bad message error marks system as unhealthy
* Finish adding test cases for changes
* Rename test file for uniqueness
* bad_message to oserror_bad_message
* Omit some checks and check for network mounts
