Mock SysLogHandler for proxy/test_server.py

Our tests get rather grabby with file descriptors, mostly due to the
SysLogHandler. In recent work I'm doing, my additional tests put
things just over the 1024 limit and tests would start to fail
somewhat randomly. This alleviates the problem by mocking out one of
the bigger users of SysLogHandler, proxy/test_server.py. If you want
to verify this has an impact, you can put the following in your
proxy/test_server.py's overall teardown method and try it with and
without the mocking:

    import subprocess
    print >>sys.stderr, '%s OPEN FILES' % len(subprocess.Popen(
        ['lsof'], stdout=subprocess.PIPE).communicate()[0].split('\n'))

Change-Id: I1bd3efe46ee69d09a32c5a964f04e36e46506446
Merge "Mock SysLogHandler for proxy/test_server.py"