Fix EZP-21964: Improve test coverage of URLAliasService

This PR resolves issue https://jira.ez.no/browse/EZP-21964

This brings line coverage of UrlAliasService to 100%

Most of the stuff was already tested but was missing complete covers annotations.
Tests for methods `load` and `removeAliases` are implemented.

Also found two bugs related to always-available aliases and showAllTranslations setting:
* https://jira.ez.no/browse/EZP-21970
* https://jira.ez.no/browse/EZP-21971

These are fixed here in separate commits.