message: [c# epoxy] Improve multiple shutdown test
  Before, the `Connection_CanBeStoppedMultipleTimes` test was just
  asserting that the connection had been shutdown once--not
  twice--before asserting.
  Now, we assert that both shutdowns have happened.
  Closes
diff: @@ -77,15 +77,12 @@ namespace UnitTest.Epoxy
  var testClientServer = await SetupTestClientServer<TestService>();
  EpoxyConnection connection = testClientServer.ClientConnection;
  - var stopTasks = new[] {connection.StopAsync(), connection.StopAsync()};
  + var stopTasks = Task.WhenAll(connection.StopAsync(), connection.StopAsync());
  var timeoutTask = Task.Delay(TimeSpan.FromSeconds(10));
  - var allTasks = new List<Task>(stopTasks) { timeoutTask };
  - var completedTask = await Task.WhenAny(allTasks);
  + var completedTask = await Task.WhenAny(stopTasks, timeoutTask);
  Assert.AreNotSame(timeoutTask, completedTask, "Timed out waiting for connection to be shutdown.");
  -
  - await connection.StopAsync();
  }
  [Test]
  