Clean up IP addresses and possibly hostnames used in the tests

[dmykarp]

mreg/tests.py and mreg/api/v1/tests.py contain lots of IP addresses and hostnames for the tests. Right now they lack consistency; it would be clearer if we only had addresses from the same private network in all the tests, both for IPv4 and IPv6.

The hostnames in the tests are now supposed to be in .example.org domain, but a check is needed to be sure we don't have a stray test hostname in another domain, also for the reason of consistency and clarity.