Use the external MgoSuite.

Mostly mechanical.
Remove the testing/mgo* bits which are now in github.com/juju/testing.
Add testing.Certs to be used when registering the MgoSuite tests.
Add testing package tests, rename getCerts.
Merge pull request #61 from frankban/external-mgosuite

Use the external MgoSuite.

Mostly mechanical.
Remove the testing/mgo* bits which are now in github.com/juju/testing.
Add testing.Certs to be used when registering the MgoSuite tests.
Destroy unit's machine if eligible.

Either way, assert that the state assumptions for being eligible or
not-eligible hold true for the transaction.