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:

- [x] remove panics
- [x] revise and cleanup
- [ ] does mocknet require cleanup? `mn.Close()`?
- [ ] when setting up mocknet, is it sufficient to link, but not connect peers?
- [ ] private key generation is really expensive/slows down tests. got any ideas for faster fixture generation that meets `mocknet` requirements? Would @whyrusleeping's parameterized version solve this problem?
This is `testutil.Peer`. The name is pretty clear.

> Why not just use PeerNetParams ?

Don't want to tie down callsites to a concrete type that's subject to change.