Replace uses of Async.RunSynchronously in test suites with an Async.RunImmediate

[dsyme]

We should replace nearly all uses of Async.RunSynchronously in our test suites with an Async.RunImmediate that never switches to a new thread.

This makes the debugging experience much, much better, including for the old VS tests and the FSharp.COmpiler.Service.Tests.

Async.RunSynchronously jumps to a new thread if not started from a thread pool thread. This is because it's trying to avoid deadlock in REPLs and other situations, however is quite conservative. For our test suites, I believe it is going to always be safe to just run on the current thread.

    type Async with
        static member RunImmediate (computation: Async<'T>, ?cancellationToken ) =
            let cancellationToken = defaultArg cancellationToken Async.DefaultCancellationToken
            let ts = TaskCompletionSource<'T>()
            let task = ts.Task
            Async.StartWithContinuations(
                computation,
                (fun k -> ts.SetResult k),
                (fun exn -> ts.SetException exn),
                (fun _ -> ts.SetCanceled()),
                cancellationToken)
            task.Result