L10n: Port GettextL10nProvider unit tests to LunrBaseTest.
L10n: Port L10n class unit tests to LunrBaseTest.
L10n: Port L10nHTMLView unit tests to LunrBaseTest.
L10n: Port L10nProvider unit tests to LunrBaseTest.
L10n: Port L10nTrait unit tests to LunrBaseTest.
L10n: Port PHPL10nProvider unit tests to LunrBaseTest.