Flaky test: DocumentIsUnloadedWhenClosed

[MikaelMayer]

Seen here: https://github.com/dafny-lang/dafny/actions/runs/10744510577/job/29801634870?pr=5754

[xUnit.net 00:06:28.67]     Microsoft.Dafny.LanguageServer.IntegrationTest.Synchronization.CloseDocumentTest.DocumentIsUnloadedWhenClosed [FAIL]
[xUnit.net 00:06:28.68]       System.AggregateException : One or more errors occurred. (Object reference not set to an instance of an object.) (A task was canceled.)
[xUnit.net 00:06:28.68]       ---- System.NullReferenceException : Object reference not set to an instance of an object.
[xUnit.net 00:06:28.68]       ---- System.Threading.Tasks.TaskCanceledException : A task was canceled.
[xUnit.net 00:06:28.68]       Stack Trace:
[xUnit.net 00:06:28.68]         
[xUnit.net 00:06:28.68]         ----- Inner Stack Trace #1 (System.NullReferenceException) -----
[xUnit.net 00:06:28.68]         D:\a\dafny\dafny\dafny\Source\DafnyLanguageServer.Test\DafnyLanguageServerTestBase.cs(89,0): at Microsoft.Dafny.LanguageServer.IntegrationTest.DafnyLanguageServerTestBase.<Initialize>b__17_0()
[xUnit.net 00:06:28.68]         D:\a\dafny\dafny\dafny\Source\DafnyLanguageServer.Test\DafnyLanguageServerTestBase.cs(102,0): at Microsoft.Dafny.LanguageServer.IntegrationTest.DafnyLanguageServerTestBase.AnonymousDisposable.Dispose()
[xUnit.net 00:06:28.68]         /_/Rx.NET/Source/src/System.Reactive/Disposables/CompositeDisposable.cs(243,0): at System.Reactive.Disposables.CompositeDisposable.Dispose()
[xUnit.net 00:06:28.68]         /_/src/xunit.execution/Extensions/ReflectionAbstractionExtensions.cs(87,0): at ReflectionAbstractionExtensions.DisposeTestClass(ITest test, Object testClass, IMessageBus messageBus, ExecutionTimer timer, CancellationTokenSource cancellationTokenSource)
[xUnit.net 00:06:28.68]         ----- Inner Stack Trace #2 (System.Threading.Tasks.TaskCanceledException) -----
[xUnit.net 00:06:28.68]         D:\a\dafny\dafny\dafny\Source\DafnyLanguageServer.Test\Util\ClientBasedLanguageServerTest.cs(256,0): at Microsoft.Dafny.LanguageServer.IntegrationTest.Util.ClientBasedLanguageServerTest.SetUp(Action`1 modifyOptions)
The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.InvalidOperationException: There is no currently active test.
   at Xunit.Sdk.TestOutputHelper.GuardInitialized() in /_/src/xunit.execution/Sdk/Frameworks/TestOutputHelper.cs:line 51
   at Xunit.Sdk.TestOutputHelper.QueueTestOutput(String output) in /_/src/xunit.execution/Sdk/Frameworks/TestOutputHelper.cs:line 66
   at DafnyCore.Test.WriterFromOutputHelper.WriteLine(String value) in D:\a\dafny\dafny\dafny\Source\DafnyCore.Test\WriterFromOutputHelper.cs:line 28
   at Microsoft.Boogie.SMTLib.SMTLibProcessTheoremProver.HandleProverError(String s)
   at Microsoft.Boogie.SMTLib.SMTLibProcess.HandleError(String msg)
   at Microsoft.Boogie.SMTLib.SMTLibProcess.SolverErrorDataReceived(Object sender, DataReceivedEventArgs e)
   at System.Diagnostics.AsyncStreamReader.FlushMessageQueue(Boolean rethrowInNewThread)