'net' module: tests and fixes

So far this updates #308 with a very basic test suite, and fixes IPv4 and/or IPv6 address detection.
Bah! Test module is a mess:
- `tt` runs 120 tests but depends on https://github.com/tessel/runtime/pull/291 which isn't merged.
- `ttt` worked with a small patch but @johnnyman727 deleted the repo
- `tinytap` only runs 112 tests, why???

(While I'm complaining anyway, none of the above test suites let me say how many tests I *expect* to run like tap would with `t.plan(N)` so it's not necessarily testing what it looks like anyway…)