sun/net/www/http/HttpClient/MultiThreadTest.java should be more resilient to unexpected traffic

Occasionally, a test server may receive traffic which does not come from its expected client. Ideally, servers should be resilient to that situation.
The MultiThreadTest.java server attempts to count the number of connections it receives. But this will not work if some other process (port scanner etc... attempts to connect to the server).
The server could be changed to ignore connections that don't come from the expected client. It can do so by verifying whether the expected URL path is present in the headers.