Improve SignalR tests to use channel

[JamesNK]

Fix race when getting activity. Using a channel gives the test the opportunity to wait for the activity to be ready.

| [0.003s] TestLifetime Information: Starting test StreamingHubMethodCreatesActivities_ReadTraceHeaders at 2024-08-02T13:53:58
| [0.007s] Microsoft.AspNetCore.SignalR.Internal.DefaultHubProtocolResolver Debug: Registered SignalR Protocol: json, implemented by Microsoft.AspNetCore.SignalR.Protocol.JsonHubProtocol.
| [0.007s] Microsoft.AspNetCore.SignalR.Internal.DefaultHubProtocolResolver Debug: Registered SignalR Protocol: messagepack, implemented by Microsoft.AspNetCore.SignalR.Protocol.MessagePackHubProtocol.
| [0.025s] Microsoft.AspNetCore.SignalR.HubConnectionHandler Debug: OnConnectedAsync started.
| [0.025s] Microsoft.AspNetCore.SignalR.Internal.DefaultHubProtocolResolver Debug: Found protocol implementation for requested protocol: json.
| [0.025s] Microsoft.AspNetCore.SignalR.HubConnectionContext Debug: Completed connection handshake. Using HubProtocol 'json'.
| [0.034s] Microsoft.AspNetCore.SignalR.Tests.HubConnectionHandlerTests Error: Test threw an exception.
| Xunit.Sdk.SingleException: The collection was expected to contain a single element, but it was empty.
|    at Xunit.Assert.GetSingleResult[T](IEnumerable`1 collection, Predicate`1 predicate, String expectedArgument) in /_/src/xunit.assert/Asserts/CollectionAsserts.cs:line 628
|    at Xunit.Assert.Single[T](IEnumerable`1 collection) in /_/src/xunit.assert/Asserts/CollectionAsserts.cs:line 588
|    at Microsoft.AspNetCore.SignalR.Tests.HubConnectionHandlerTests.StreamingHubMethodCreatesActivities_ReadTraceHeaders() in /_/src/SignalR/server/SignalR/test/Microsoft.AspNetCore.SignalR.Tests/HubConnectionHandlerTests.Activity.cs:line 258
|    at Xunit.Sdk.TestInvoker`1.<>c__DisplayClass48_0.<<InvokeTestMethodAsync>b__1>d.MoveNext() in /_/src/xunit.execution/Sdk/Frameworks/Runners/TestInvoker.cs:line 264
| --- End of stack trace from previous location ---
|    at Xunit.Sdk.ExecutionTimer.AggregateAsync(Func`1 asyncAction) in /_/src/xunit.execution/Sdk/Frameworks/ExecutionTimer.cs:line 48
|    at Xunit.Sdk.ExceptionAggregator.RunAsync(Func`1 code) in /_/src/xunit.core/Sdk/ExceptionAggregator.cs:line 90
| [0.037s] TestLifetime Information: Finished test StreamingHubMethodCreatesActivities_ReadTraceHeaders in 0.0335153s