'net' module: tests and fixes

So far this updates #308 with a very basic test suite, and fixes IPv4 and/or IPv6 address detection.
Please see https://github.com/tcr/tinytap/issues/4 — I tried switching to tinytap but it only ran like half my tests so I went back to 'ttt'. But I guess the package.json ended up wrong.

Looks like https://github.com/tessel/runtime/pull/279 is merged though so I will update to get rid of the relative path hack (and rebase to master again while I'm at it).