Improve test coverage

[aramperes]

Placeholder ticket to remind me to go back to write some unit tests in parts where it would be useful.

Mainly:

• testing the virtual interface logic in isolation
• testing port pools in isolation

I would also like to have a suite of integration tests that use a real WireGuard network and stress-test onetun in various scenarios. It gets tiring to run these experiments manually and I can't possibly check everything every time.