Expose test comparers so other Razor projects can use them.

[NTaylorMullen]

Current Microsoft.AspNet.Tooling.Razor is using non-test variations of Razor's TagHelperDescriptor comparers. Instead of duplicating all the code for these comparers we should move the current comparers to a Microsoft.AspNet.Razor.Test.Sources project in /src (so a package is created). Microsoft.AspNet.Razor.Test.Sources should be a sources package that does not publish packages externally; this way we can use the common Razor test utilities across our repos but not impact external consumers.

[NTaylorMullen]

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