Fix StreamPool_MultipleStreamsConcurrent_StreamsReturnedToPool (#57473)

Wait for streams to complete before triggering a pool update since they won't be eligible for reuse if they haven't completed.

For #39477

Author: amcasey

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

@@ -474,6 +474,8 @@ await ExpectAsync(Http2FrameType.DATA,
             withFlags: (byte)Http2DataFrameFlags.END_STREAM,
             withStreamId: 3);
 
+        await WaitForAllStreamsAsync().DefaultTimeout();
+
         // TriggerTick will trigger the stream to be returned to the pool so we can assert it
         TriggerTick();