Bug 1043643 - Use shared mock_l10n in all Settings test

This is a followup landing to https://github.com/mozilla-b2g/gaia/commit/a6c29cc2f048dc08a2f008322699368b7e5cd9b0.
Merge pull request #24931 from stasm/1043643-mockl10n-followup

Bug 1043643 - Use shared mock_l10n in all Settings test