[System.Web] Tests: Add a timeout for WaitOne () calls

This should get rid of the test hangs we're seeing on monojenkins (although it doesn't yet fix the underlying problem).

Note: before this fix, I could reproduce the hang once with the following GDB output: http://pastebin.com/pUnzQcJ3

@alexrp: does it make sense that the hangs are caused by the WaitOne calls or am I misinterpreting the GDB output?