Simplify weirdly broken test code

[djc]

#871 seems to have added a test with this code:

    let next_retire_prior_to = active_cid_num + 1;
    pair.client_conn_mut(client_ch).ping();
    // Server retires all valid remote CIDs
    pair.server_conn_mut(server_ch)
        .rotate_local_cid(next_retire_prior_to);
    pair.drive();
    assert!(!pair.client_conn_mut(client_ch).is_closed());
    assert!(!pair.server_conn_mut(server_ch).is_closed());
    assert_matches!(
        pair.client_conn_mut(client_ch).active_rem_cid_seq(),
        _next_retire_prior_to
    );

Note on the last line that _next_retire_prior_to is a new identifier, which does not actually reference the earlier binding next_retire_prior_to. Because this is is assert_matches!(), I think the intended semantics here are a little unclear, and this was probably be intended as assert_eq!(.., next_retire_prior_to).

[gretchenfrage]

Rebased and resolved merge conflict.