runtests: set web.ipv6 if we use IPv6 As explained by the previous patch, we need to set "web.ipv6=True" if we decide to use IPv6. Otherwise "hg serve" will still try to listen on IPv4. This patch makes it so by appending web.ipv6 to "extra configs". This patch was tested in a Linux system with IPv6, by the following steps: 1. Change hgweb/server.py temporarily to write a file if IPv6HTTPServer.__init__ is called. 2. run-tests.py -l --keep-tmpdir test-serve.t 3. Check the generated .hgrc, make sure it sets web.ipv6=1. 4. Check the log file to make sure IPv6HTTPServer.__init__ is called.