Decrease number of socket instances in SocketPerformance_MultipleSocketClientAsync_LocalHostServerAsync

Larger number causes listen queue to be exceeded, resulting in client failures due to connection being rejected by server.

Author: stephentoub

src/System.Net.Sockets/tests/PerformanceTests/SocketPerformanceAsyncTests.cs

@@ -37,7 +37,7 @@ public void SocketPerformance_MultipleSocketClientAsync_LocalHostServerAsync()
             SocketImplementationType clientType = SocketImplementationType.Async;
             int iterations = 200 * _iterations;
             int bufferSize = 256;
-            int socket_instances = 200;
+            int socket_instances = 20;
 
             var test = new SocketPerformanceTests(_log);