Add parts kit tests

[davist11]

• Generating the correct nav
• Can grab a craft asset
• Is a parts kit request
• Correctly selects dark theme config if 'theme' => 'dark' is passed
• Correctly outputs custom theme CSS

[davist11]

Was having issues detecting whether it was a parts kit request because the segments aren't set when mocking the URL in the unit test.

Also tried testing if parts-kit.css or parts-kit.js was $I->see() in a functional test, but couldn't access stuff in the head or JS