test_options_unixsocket_configfile: Don't use argv

Without this, if you invoke a test command that has command-line options, `supervisor.options` will try to parse those options and can fail because of unrecognized options:

    $ .tox/py27/bin/py.test -v supervisor/tests/test_options.py -k test_options_unixsocket_configfile
    ...
    self = <supervisor.options.ClientOptions instance at 0x103aeb050>,
    msg = GetoptError('option -v not recognized', 'v')

        def usage(self, msg):
            """Print a brief error message to stderr and exit(2)."""
            self.stderr.write("Error: %s\n" % str(msg))
            self.stderr.write("For help, use %s -h\n" % self.progname)
    >       self.exit(2)
    E       SystemExit: 2

    supervisor/options.py:143: SystemExit
    -----------------------------------------------------------------------------
    Captured stderr call
    -----------------------------------------------------------------------------
    Error: option -v not recognized
    ...

Changed `test_options_unixsocket_configfile` to explictly set `arg` to an empty tuple so that `*arg is not None` and it doesn't pull args from `sys.argv`.

Output:

    $ .tox/py27/bin/py.test -v supervisor/tests/test_options.py -k test_options_unixsocket_configfile
    ...
    supervisor/tests/test_options.py::ClientOptionsTests::test_options_unixsocket_configfile PASSED
    ...