'net' module: tests and fixes

So far this updates #308 with a very basic test suite, and fixes IPv4 and/or IPv6 address detection.
@natevw, `tinytap` only runs 112 tests when you run `npm test` or `tessel run <TESTFILE>`? I believe `tinytap` has different log levels so if you run `npm test`, the number of tests it reports is related to the number of files it's testing.