(maint) Refacter `parse_inet_address` to improve readability

Prior to this commit, `Facter::Util::IP.parse_inet_address` used a long,
cumbersome regex to verify valid IPv4 IP addresses. It also used long,
cumbersome strings as test output. This commit replaces the regex with
the IPAddr class. It also puts the long cuumbersome output strings into
a class for modularity, formatting them as HEREDOCs for improved
readability.