Tests for corner-case behavior of relay selection functions

Arti !2880 (merged) introduced several corner-case workarounds for NetDir::pick_relay and pick_n_relays. These are not necessarily in their final form, but it might be a good idea to have a test or two to make sure that they behave as expected. (In arti #1907, we are trying to find the root cause here, and might revert some of the workarounds.)

We should definitely add tests once the behavior is settled.

cc @jnewsome