Add NavbarHorizontalTest

- Add NavbarHorizontalTest to to check on the [0] issue
- Fix array_map( trim, ...) to array_map( 'trim', ...)
- Move Xml file bulk load to XmlFileProvider
- XmlFileProvider be used by XmlLayoutFileValidityTest and NavbarHorizontalTest
- Add bootstrap to register classes using PSR-4

[0] https://www.mediawiki.org/wiki/Thread:Skin_talk:Chameleon/In_Navbar_how_to_display_a_single_link_instead_of_a_dropdown_%3F/reply_%284%29

Change-Id: I551a9c6831c6cf6075b5751d843390816e07634b