runtests: export HGIPV6 to hint test scripts whether to use IPv6 Previously, run-tests.py only exports HGPORT, and scripts in tests do not know if IPv6 should be used. And that breaks scripts like dumbhttp.py which always uses IPv4. This patch makes run-tests.py export HGIPV6, which can help test scripts like dumbhttp.py and tinyproxy.py to decide whether to use IPv6 or not.