Add tests for the cohost linked editing range endpoint

[davidwengier]

Part of #9519 and #10603

Finally some cohosting tests! Some people would say that I'm being lazy in adding tests for the simplest endpoint we have. Those people have a point.

The test infra here almost entirely avoids ServiceHub, and certainly avoids the Roslyn solution sync mechanism, but it does use our real services and service factories, including the OOP services' separate MEF composition, so the services themselves are partying on a real Roslyn (test) solution and are using real implementations of all of their dependencies.

Things not covered by this test infra yet:

• OOP initialization
• Adding generated C# files to the Roslyn solution (IDynamicFile does this in real life)
• Dealing with Html documents in any way

Future PRs to add tests for more endpoints should add these as needed.

[DustinCampbell]

It's not worth changing anything, but I wanted to mention that there's a handy mock helper for creating IServiceProvider: VsMocks.CreateServiceProvider(...).

[davidwengier]

When its this easy to do, I'd rather avoid a mock 😛

[DustinCampbell]

I wonder if we'll be breaking out another test library later, since MS.CA.Remote.Razor should never be loaded in Visual Studio.

[davidwengier]

I considered doing this as a separate project now, but it didn't seem to add much value. Certainly in future we'll need to as the cohosting bits will have to move somewhere other than the VS layer, so they can work in VS Code. Once that happens we'll definitely need something new, we just don't know what it looks like yet.

[davidwengier]

Have responded to the feedback here, thanks very much. Going to merge this as is for now, just to keep the code flowing. Lots more to come though, outlined in #10603, so I suspect some of this infra will change soon enough.