[HTTP] Fix test race with semaphore.

src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientTest.cs

@@ -1117,10 +1117,10 @@ await IgnoreExceptions(async () =>
 
         [Fact]
         [OuterLoop]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/39056")]
         [SkipOnPlatform(TestPlatforms.Android, "Synchronous Send is not supported on Android")]
         public async Task Send_TimeoutRequestContent_Throws()
         {
+            var semaphore = new SemaphoreSlim(0);
             await LoopbackServer.CreateClientAndServerAsync(
                 async uri =>
                 {
@@ -1146,12 +1146,14 @@ await LoopbackServer.CreateClientAndServerAsync(
 
                     TaskCanceledException ex = await Assert.ThrowsAsync<TaskCanceledException>(() => sendTask);
                     Assert.IsType<TimeoutException>(ex.InnerException);
+                    semaphore.Release();
                 },
                 async server =>
                 {
                     await server.AcceptConnectionAsync(async connection =>
                     {
                         await IgnoreExceptions(connection.ReadRequestDataAsync());
+                        await semaphore.WaitAsync();
                     });
                 });
         }