Fix Socket.SendFile test: SliceBuffers_Success (#48457)

* do not expect a single receive

* use expected.Length

* enable test

Author: antonfirsov

src/libraries/System.Net.Sockets/tests/FunctionalTests/SendFile.cs

@@ -202,7 +202,6 @@ public async Task NoFile_Succeeds(bool usePreBuffer, bool usePostBuffer)
         }
 
         [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/47734")]
         public async Task SliceBuffers_Success()
         {
             if (!SupportsSendFileSlicing) return; // The overloads under test only support sending byte[] without offset and length
@@ -215,17 +214,25 @@ public async Task SliceBuffers_Success()
             rnd.NextBytes(postBuffer);
 
             byte[] expected = preBuffer.ToArray().Concat(postBuffer.ToArray()).ToArray();
+            uint expectedChecksum = Fletcher32.Checksum(expected, 0, expected.Length);
 
             (Socket client, Socket server) = SocketTestExtensions.CreateConnectedSocketPair();
 
             using (client)
             using (server)
             {
                 await SendFileAsync(client, null, preBuffer, postBuffer, TransmitFileOptions.UseDefaultWorkerThread);
-                byte[] receiveBuffer = new byte[100];
-                int receivedBytes = server.Receive(receiveBuffer);
-                Assert.Equal(100, receivedBytes);
-                AssertExtensions.SequenceEqual(expected, receiveBuffer);
+                Fletcher32 receivedChecksum = new Fletcher32();
+                byte[] receiveBuffer = new byte[expected.Length];
+                int receivedBytes;
+                int totalReceived = 0;
+                while (totalReceived < expected.Length && (receivedBytes = server.Receive(receiveBuffer)) != 0)
+                {
+                    totalReceived += receivedBytes;
+                    receivedChecksum.Add(receiveBuffer, 0, receivedBytes);
+                }
+                Assert.Equal(expected.Length, totalReceived);
+                Assert.Equal(expectedChecksum, receivedChecksum.Sum);
             }
         }