Fix SocketsHttpHandlerTest_HttpClientHandlerTest_Http3.ReadAsStreamAsync_Cancellation data race

Author: rzikm

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.cs

@@ -6,6 +6,7 @@
 using System.Linq;
 using System.Net.Http.Headers;
 using System.Net.Sockets;
+using System.Net.Quic;
 using System.Net.Test.Common;
 using System.Security.Authentication;
 using System.Security.Cryptography;
@@ -1396,6 +1397,10 @@ await server.AcceptConnectionAsync(async connection =>
                         await connection.SendResponseAsync(HttpStatusCode.OK, headers: new HttpHeaderData[] { new HttpHeaderData("Transfer-Encoding", "chunked") }, isFinal: false);
                         await connection.SendResponseBodyAsync("1\r\nh\r\n", false);
                     }
+                    catch (QuicException ex) when (ex.ApplicationErrorCode == 0x10c /*H3_REQUEST_CANCELLED*/)
+                    {
+                        // The request was cancelled before we sent the body, ignore
+                    }
                     catch (IOException ex)
                     {
                         // when testing in the browser, we are using the WebSocket for the loopback