Futher improve the test coverage

[maqi]

The current coverage report only shows a test coverage of 50%.

We may need more tests to cover changing membership, but with simple net setup and not requiring qp2p.

as a note, the previous tests using members doesn't contribute to the test coverage.

For reference, we could have a look at https://github.com/dan-da/brb/blob/master/src/net.rs.
With some usage/tests are in: https://github.com/dan-da/brb_dt_orswot/blob/master/tests/brb_orswot_net.rs
for a fake network testing.