Reimplement and reenable DebugServiceTests.cs

* Removed uncessary using directives and sorted
* Applied `readonly` and `const` suggestions
* Updated use of `psesHost` instead of `powerShellContextService`
* Call `GC.SuppressFinalize()` in `Dispose()` to avoid warning
* Improved names of things
* Removed unnecessary `this.` prefixes
* Added easy-to-use PowerShell execution wrappers
* Removed all logic around `sessionStateQueue` because of new pipeline
* Fixed threading assumptions by using `ConfigureAwait(true)`
* Used `Task.Run()` to avoid deadlock when calling onto pipeline thread
* Replaced `FirstOrDefault` with `Array.Find`
* Removed deprecated data from `DebuggerAcceptsScriptArgs`
* Stopped aborting at end of each test because xUnit disposes everything
* Ditto for waiting on `executeTask`
* Skip broken tests around setting variables (punting this fix)
* General reorginzation and cleanup

Author: andyleejordan