Update Http2_FlowControl_ClientDoesNotExceedWindows to use ByteAtATim…

…eContent for compat with recent buffering change. (dotnet/corefx#39472)

Clear ActiveIssue as original issue in dotnet/corefx#38799 could not be reproduced.

Commit migrated from dotnet/corefx@929bd81

src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.Http2.cs

@@ -1163,15 +1163,14 @@ private static async Task<int> ReadToEndOfStream(Http2LoopbackConnection connect
  return bytesReceived;
  }
 
- [ActiveIssue(38799)]
  [OuterLoop("Uses Task.Delay")]
  [ConditionalFact(nameof(SupportsAlpn))]
  public async Task Http2_FlowControl_ClientDoesNotExceedWindows()
  {
  const int InitialWindowSize = 65535;
  const int ContentSize = 100_000;
 
- var content = new ByteArrayContent(TestHelper.GenerateRandomContent(ContentSize));
+ var content = new ByteAtATimeContent(ContentSize);
 
  using (var server = Http2LoopbackServer.CreateServer())
  using (HttpClient client = CreateHttpClient())