Add code generation tests tests for @inherits directive in Razor.Extensions

[ajaybhargavb]

Currently we don't have any code generation tests that use @inherits directive.

cc' @NTaylorMullen

[NTaylorMullen]

There's also a possible tooling impact issue here with @inherits and TModel. Our ModelDirective bits update the @inherits token which will break IntelliSense in the editor:

Razor/src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ModelDirective.cs

Line 77 in 56c2d76

tokens[0].Content = baseType;