Fix notification handler tests to use per server task completion instances

[dibarbet]

Reproduced locally using multiple iterations. Issue was the static task completion source was being re-used across multiple server instances, causing the test to exit too early after a previous test completed.

Fixes failures like https://dnceng-public.visualstudio.com/public/_build/results?buildId=636591&view=ms.vss-test-web.build-test-results-tab&runId=15674606&resultId=230727&paneView=dotnet-dnceng.dnceng-anon-build-release-tasks.helix-anon-test-information-tab

[06:38:20.827][Info]Using C# from request text document
 [06:38:20.827][Info]Shutting down
 [06:38:20.827][Error]Could not find appropriate workspace or solution on NotificationHandler
 [06:38:20.827][Start]NotificationHandler
 [06:38:20.827][Warning]
 System.InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed.
    at System.Threading.Tasks.TaskCompletionSource`1.SetResult(TResult result)
    at Microsoft.CodeAnalysis.LanguageServer.UnitTests.HandlerTests.NotificationHandler.HandleNotificationAsync(TestRequestTypeOne request, RequestContext context, CancellationToken cancellationToken) in /_/src/Features/LanguageServer/ProtocolUnitTests/HandlerTests.cs:line 200
    at Microsoft.CommonLanguageServerProtocol.Framework.QueueItem`3.<StartRequestAsync>d__20.MoveNext() in /_/src/Features/LanguageServer/Microsoft.CommonLanguageServerProtocol.Framework/QueueItem.cs:line 162
 [06:38:20.827][End]NotificationHandler

[ToddGrun]

[CyrusNajmabadi]

signing off as this is all test-only changes.

[CyrusNajmabadi]

Dumb question, why do we have the test handlers here instead of tewstnig the real handler?

[dibarbet]

Dumb question, why do we have the test handlers here instead of tewstnig the real handler?

A few reasons

1. So the test can wait and verify that a notification handler is actually called (requires the task completion source stuff inside the handler here since the caller's task completes before the handler is executed).
2. Isolation against changes - these are intentionally testing that certain shapes of the handlers work (for example notifications without params, notifications with params). Avoids issues where the real handler might add/remove parameters for whatever reason.
3. I don't think we actually have a real parameterless notification handler (but if we, or a consumer ever wants to add one, it needs to work).