Fix flakey HTTP/2 tests (#39588)

Author: JamesNK

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http2/Http2ConnectionTests.cs

@@ -5219,7 +5219,7 @@ await WaitForConnectionErrorAsync<Http2ConnectionErrorException>(
     [Fact]
     public async Task StartConnection_SendPreface_ReturnSettings()
     {
-        await InitializeConnectionWithoutPrefaceAsync(_noopApplication);
+        InitializeConnectionWithoutPreface(_noopApplication);
 
         await SendAsync(Http2Connection.ClientPreface);
 
@@ -5234,7 +5234,7 @@ await ExpectAsync(Http2FrameType.SETTINGS,
     [Fact]
     public async Task StartConnection_SendHttp1xRequest_ReturnHttp11Status400()
     {
-        await InitializeConnectionWithoutPrefaceAsync(_noopApplication);
+        InitializeConnectionWithoutPreface(_noopApplication);
 
         await SendAsync(Encoding.ASCII.GetBytes("GET / HTTP/1.1\r\n"));
 
@@ -5247,7 +5247,7 @@ public async Task StartConnection_SendHttp1xRequest_ReturnHttp11Status400()
     [Fact]
     public async Task StartConnection_SendHttp1xRequest_ExceedsRequestLineLimit_ProtocolError()
     {
-        await InitializeConnectionWithoutPrefaceAsync(_noopApplication);
+        InitializeConnectionWithoutPreface(_noopApplication);
 
         await SendAsync(Encoding.ASCII.GetBytes($"GET /{new string('a', _connection.Limits.MaxRequestLineSize)} HTTP/1.1\r\n"));
 
@@ -5267,7 +5267,7 @@ public async Task StartTlsConnection_SendHttp1xRequest_NoError()
         tlsHandshakeMock.SetupGet(m => m.Protocol).Returns(SslProtocols.Tls12);
         _connection.ConnectionFeatures.Set<ITlsHandshakeFeature>(tlsHandshakeMock.Object);
 
-        await InitializeConnectionWithoutPrefaceAsync(_noopApplication);
+        InitializeConnectionWithoutPreface(_noopApplication);
 
         await SendAsync(Encoding.ASCII.GetBytes("GET / HTTP/1.1\r\n"));
 
@@ -5277,7 +5277,7 @@ public async Task StartTlsConnection_SendHttp1xRequest_NoError()
     [Fact]
     public async Task StartConnection_SendNothing_NoError()
     {
-        await InitializeConnectionWithoutPrefaceAsync(_noopApplication);
+        InitializeConnectionWithoutPreface(_noopApplication);
 
         await StopConnectionAsync(expectedLastStreamId: 0, ignoreNonGoAwayFrames: false);
     }

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http2/Http2TestBase.cs

@@ -470,7 +470,7 @@ protected void CreateConnection()
         _timeoutControl.Initialize(_serviceContext.SystemClock.UtcNow.Ticks);
     }
 
-    protected async Task InitializeConnectionWithoutPrefaceAsync(RequestDelegate application)
+    protected void InitializeConnectionWithoutPreface(RequestDelegate application)
     {
         if (_connection == null)
         {
@@ -493,14 +493,15 @@ async Task CompletePipeOnTaskCompletion()
         }
 
         _connectionTask = CompletePipeOnTaskCompletion();
-
-        // Lose xUnit's AsyncTestSyncContext so middleware always runs inline for better determinism.
-        await ThreadPoolAwaitable.Instance;
     }
 
     protected async Task InitializeConnectionAsync(RequestDelegate application, int expectedSettingsCount = 3, bool expectedWindowUpdate = true)
     {
-        await InitializeConnectionWithoutPrefaceAsync(application);
+        InitializeConnectionWithoutPreface(application);
+
+        // Lose xUnit's AsyncTestSyncContext so middleware always runs inline for better determinism.
+        await ThreadPoolAwaitable.Instance;
+
         await SendPreambleAsync();
         await SendSettingsAsync();