Improve test coverage of network address functions

Improve test coverage of network address functions

The following functions were not covered by any tests:
- abbrev(inet)
- set_masklen(cidr)
- set_masklen(inet)
- netmask(inet)
- hostmask(inet)

While on it, this improves the output of some of the existing queries in
the test inet to use better aliases.

Author: Aleksander Alekseev
Reviewed-by: Jacob Champion, Keisuke Kuroda, Tom Lane
Discussion: https://postgr.es/m/CAJ7c6TOyZ9bGNrDK6Z3Q0gr9ow8ZpOm+=+01mpE0dsdH4C+u9A@mail.gmail.com