Add tests to validate @removetaghelper functionality.

- Added utility methods to construct valid SyntaxTreeNodes that represent the @removetaghelper directive.
- Added parse level unit tests to validate the @removetaghelper generates an accurate SyntaxTreeNode.
- Added parse level unit tests to validate the @removetaghelper throws with bad formats.
- Added TagHelperRegistration unit tests to validate the RemoveTagHelperCodeGenerators are understood and affect the descriptors found.
- Added Designtime mapping tests to validate correct source mappings are made to ensure proper coloring and lack-of C# intellisense.
- Added end-to-end tests to validate @removetaghelper can essentially disable TagHelpers on a page.

#112 

Split out implementation since this was a rather large PR: https://github.com/aspnet/Razor/pull/204

/cc @pranavkm