Increase delay in test for GH Actions CTR

The test seems to be flaky on GH Actions which is probably caused by the
environment simply being too slow to send the 3 expected messages in
200 ms in the background.
So, I'm doubling the timeout to 400 ms as a first step.

gremlin-dotnet/test/Gremlin.Net.UnitTest/Driver/ConnectionTests.cs

@@ -250,7 +250,7 @@ public async Task ShouldContinueSubmittingOtherMessagesIfOneIsCancelled()
 
             await Assert.ThrowsAsync<TaskCanceledException>(async () => await taskToCancel);
             Assert.True(taskToCancel.IsCanceled);
-            await Task.Delay(TimeSpan.FromMilliseconds(200)); // wait a bit to let the messages being sent
+            await Task.Delay(TimeSpan.FromMilliseconds(400)); // wait a bit to let the messages being sent
             await mockedClientWebSocket.Received(3).SendAsync(Arg.Any<ArraySegment<byte>>(),
                 Arg.Any<WebSocketMessageType>(), Arg.Any<bool>(), Arg.Any<CancellationToken>());
         }