[System] Use the same port number instead of a random port in HttpWebRequestTest

A random port doesn't add any value and resulted in at least one port conflict on monojenkins:
http://monojenkins.cloudapp.net/job/Mono/243/testReport/MonoTests.System.Net/HttpWebRequestTest/BeginGetRequestStreamCancelIfNotAllBytesWritten/

mcs/class/System/Test/System.Net/HttpWebRequestTest.cs

@@ -2319,7 +2319,7 @@ namespace MonoTests.System.Net
 
 		void DoRequest (Action<HttpWebRequest, EventWaitHandle> request)
 		{
-			int port = rand.Next (20000, 65535);
+			int port = 30158;
 
 			ManualResetEvent completed = new ManualResetEvent (false);
 			Uri address = new Uri (string.Format ("http://localhost:{0}", port));
@@ -2333,7 +2333,7 @@ namespace MonoTests.System.Net
 
 		void DoRequest (Action<HttpWebRequest, EventWaitHandle> request, Action<HttpListenerContext> processor)
 		{
-			int port = rand.Next (20000, 65535);
+			int port = 30158;
 
 			ManualResetEvent [] completed = new ManualResetEvent [2];
 			completed [0] = new ManualResetEvent (false);