testing: drop databases before each Mgo test
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

reedobrien/store