runtests: set web.ipv6 if we use IPv6...
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.
