Move Microsoft.Framework.WebEncoders.Testing to this repo

[Eilon]

It's in https://github.com/aspnet/Testing/ right now.

There are currently some circular references in the repo-to-repo dependencies that we'd like to fix, and this is one of them. The idea of putting this package here is that if you're referencing the regular runtime package, you may very well want the companion test-helper package as well.

Note: the package needs to stay in the /src folder because this is a "runtime" package, even if it's test helpers.

[Eilon]

Related: aspnet/Logging#252

[Tratcher]

Obsolete due to #391 ?

Also, the Testing package used to be in this repo and was moved for some reason. Need to track that down before moving it back.

[Eilon]

No we still need the test helpers regardless of where the actual encoding logic lives.

Hmm I don't recall why it was ever moved... maybe just to group all the test stuff in one place?