Testing new Language Server

[DonJayamanne]

@MikhailArkhipov You might want to keep an eye on the following repo https://github.com/octref/vscode-language-server-e2e-test

Perhaps we can reuse something from there.

[MikhailArkhipov]

PTVS repo has bunch of LS tests we just need to separate test utility project into Windows/Core parts and add .NET Core tests to the LS project per microsoft/PTVS#4185.

However, the above may come useful as a semi-integration test.