Add tests for taxonomy term handling

This refactors MetatagTestHelper a little to add some methods for creating various entity types and data structures, moves MetatagUnitTest to run after the test helper, then MetatagNodeUITest, and adds MetatagTermUITest, which is based upon MetatagNodeUITest. Some refactoring was done in MetatagNodeUITest (and MetatagTermUITest) to make it easier to work with, and some additional assertions have been added to verify that the requested pages are actually loaded. I've also updated MetatagContextTestCase to run off MetatagTestHelper and adjusted it accordingly.

Committed. This will help going forwards.