Fixed flaky longpolling tests (#1993)

- Wait for disposal and removal of the connection

src/Microsoft.AspNetCore.Http.Connections/HttpConnectionContext.cs

@@ -79,6 +79,9 @@ namespace Microsoft.AspNetCore.Http.Connections
 
         public SemaphoreSlim Lock { get; } = new SemaphoreSlim(1, 1);
 
+        // Used for testing only
+        internal Task DisposeAndRemoveTask { get; set; }
+
         public Task TransportTask { get; set; }
 
         public Task ApplicationTask { get; set; }

src/Microsoft.AspNetCore.Http.Connections/HttpConnectionDispatcher.cs

@@ -516,8 +516,9 @@ namespace Microsoft.AspNetCore.Http.Connections
             // Complete the receiving end of the pipe
             connection.Application.Output.Complete();
 
-            // Dispose the connection gracefully, but don't wait for it
-            _ = _manager.DisposeAndRemoveAsync(connection, closeGracefully: true);
+            // Dispose the connection gracefully, but don't wait for it. We assign it here so we can wait in tests
+            connection.DisposeAndRemoveTask = _manager.DisposeAndRemoveAsync(connection, closeGracefully: true);
+
             context.Response.StatusCode = StatusCodes.Status202Accepted;
             context.Response.ContentType = "text/plain";
         }

test/Microsoft.AspNetCore.Http.Connections.Tests/HttpConnectionDispatcherTests.cs

@@ -1709,13 +1709,17 @@ namespace Microsoft.AspNetCore.Http.Connections.Tests
 
                 await dispatcher.ExecuteAsync(deleteContext, options, app).OrTimeout();
 
+                // Verify the response from the DELETE request
+                Assert.Equal(StatusCodes.Status202Accepted, deleteContext.Response.StatusCode);
+                Assert.Equal("text/plain", deleteContext.Response.ContentType);
+
                 // Verify that everything shuts down
                 await connection.ApplicationTask.OrTimeout();
                 await connection.TransportTask.OrTimeout();
 
-                // Verify the response from the DELETE request
-                Assert.Equal(StatusCodes.Status202Accepted, deleteContext.Response.StatusCode);
-                Assert.Equal("text/plain", deleteContext.Response.ContentType);
+                Assert.NotNull(connection.DisposeAndRemoveTask);
+
+                await connection.DisposeAndRemoveTask.OrTimeout();
 
                 // Verify the connection was removed from the manager
                 Assert.False(manager.TryGetConnection(connection.ConnectionId, out _));