Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
don't use abbreviations even in test code.  readers have to stop and figure them out.  so `myTagHelperBoundPI` -> `propertyInfo` and `bpi` -> `mock` or perhaps `propertyInfoMock` and `propertyInfo`.  I prefer the first but either is fine.