Add synchronization into the test

Author: ManickaP

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

@@ -2173,6 +2173,7 @@ public async Task SendAsync_InvalidRequestUri_Throws()
         public async Task SendAsync_RequestWithDangerousControlHeaderValue_ThrowsHttpRequestException(char dangerousChar, HeaderType headerType)
         {
             TaskCompletionSource<bool> acceptConnection = new TaskCompletionSource<bool>();
+            SemaphoreSlim clientFinished = new SemaphoreSlim(0);
 
             await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>
             {
@@ -2216,6 +2217,7 @@ await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>
                     // WinHTTP validates the input before opening connection whereas SocketsHttpHandler opens connection first and validates only when writing to the wire.
                     acceptConnection.SetResult(!IsWinHttpHandler);
                     var ex = await Assert.ThrowsAnyAsync<Exception>(() => client.SendAsync(request));
+                    clientFinished.Release();
                     if (IsWinHttpHandler)
                     {
                         var fex = Assert.IsType<FormatException>(ex);
@@ -2232,7 +2234,13 @@ await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>
             {
                 if (await acceptConnection.Task)
                 {
-                    await IgnoreExceptions(server.AcceptConnectionAsync(c => Task.CompletedTask));
+                    await IgnoreExceptions(async () =>
+                    {
+                        await server.AcceptConnectionAsync(async _ =>
+                        {
+                            await clientFinished.WaitAsync(TestHelper.PassingTestTimeout);
+                        });
+                    });
                 }
             });
         }