[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/
Merge pull request #1123 from akoeplinger/fix-webreq-test

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