Added test for testing on("complete") returned value "last_seq"

From a IRC today with  @nolanlawson i've created my first pouch test, for a weird case where on('complete') on a given replication sequence, returns a value on last_seq, which is altered after the event itself, and then db.info() later returns a different (incremented) update_seq.

Please be kind, it is my first code for the project.

Cheers.