test: add get network group tests to tests/net

opening this issue to track the request from here: https://github.com/handshake-org/hsd/pull/217

That PR includes all of the "static" tests to network addresses' getGroup() function. While that is a good start, it might be prudent for us to eventually build a test that ensures we are not adding 2 peers who exist in the same network group to our outbound connections. This requires more of a live simulation environment than what is provided in the current net address tests.

Start in tests/net and review the static network-address getGroup() tests from PR #217. Then determine how the networking tests can provide a live simulation of outbound connections. Done means coverage showing that two peers from the same network group are not both added to outbound connections.

Written by the indexing model from the issue text.