use `net/mocknet` in integration tests

This PR adds the `net/mocknet` to the tests/benchmarks that perform the Add/Cat operation.

@jbenet @whyrusleeping 

TODOs:

- [ ] remove panics
- [ ] revise and cleanup
- [ ] does mocknet require cleanup? `mn.Close()`?
@maybebtc 

- `Link` means that a connection is **possible**. (think physical link)
- `Connect` means "open a connection". (think a `net.Conn`)

So yeah, you dont _have to_ link nodes together, if someone will try to find the node and `Dial` as the result of some other actions in the test.