runtests.pl: fix startup of IPv6 servers

Commit curl-7_23_1-143-g82180643

 changed the parameter of
responsive_http_server to accept types other than IPv6 (converting
from a boolean to a string), but only considered the lower-case "ipv6"
and not the "IPv6" variant. This caused all servers to start in IPv4
mode instead.

This patch converts the remaining cases to "ipv6". While not strictly
necessary for the run*server variants, these got also converted for
consistency and to prevent future errors.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>