[r=axwalk],[bug=1318527] testing: drop databases before each Mgo test
Previously the store package had its own mgo
suite, which was largely the same as the one
in juju-core/testing. It also had code to drop
the non-admin databases before each test case,
which I did not carry over when migrating. This
caused flakeyness.
Fixes lp:1318527
https://codereview.appspot.com/94460045/