(maint) Refactor `.parse_inet_address` and add fictures for testing

Prior to this commit, `.parse_inet_address` was not ass readable as it
could be, and test `ifconfig` data was in a class. This commit makes
`ifconfig` more readable and puts the test `ifconfig` data in fixtures
(maint) Remove old code from `ip_spec.rb`

Prior to this commit, the TestOutput class (which was made unnecessary
by the previous commit) had not been removed. This commit removes it.