don't trigger update from checkPassword, it is already  called by userExists, this is enough.

do not write to database when the value is the same

add test cases. also split test classes in a file each, looks like only the first class is being executed

also appconfig shall not write to database if the value is unchanged

On SQLite the app order can be arbitrary and cause strange bugs.
On MySQL, the app order seems to be always alphabetical.

This fix enforces alphabetical order to make sure that all environments
behave the same and to reduce bugs related to app loading order.

Fixes #6442

Implemented unittest for OC\AppConfig

This reverts commit c74f3d0b.