EZP-21713: Added extension/*/tests/settings to override dirs for Unit Tests

When unit-testing extensions, it is currently not possible to override settings from within the extension only for the tests. This is currently only possible when we put the settings into `/ezpublish_root/tests/settings/`.

With this PR, it is possible to put settings into `extension/<my_extension>/tests/settings` which will then be used during Unit Tests (and only then).

Cheers
:octocat: Jérôme

https://jira.ez.no/browse/EZP-21713