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()`?
It's necessary to link peers, but _not_ necessary to connect them, right?

@jbenet