add suite of ipv6 formatting tests, related to #796

[mzpqnxow]

I split out the oddly named test function for address formatting (named for ipv6, but had only a single test case, for ipv4)

I did this when trying to track down #796

Unfortunately, I didn't find any issues .... if anyone wants to take a look, feel free. I think more information is needed to reproduce the behavior that is reported there