Drop parallel test databases before running create

If the database isn't dropped in full then tables dropped in a migration
will still exist in the parallel databases.

Because the primary database was being dropped before creation this lead
to intermittent false positives where a test would fail if run against
the primary but would pass if run against one of the parallels where the
table still exists.
Merge pull request #1330 from alphagov/make-jenkins-drop-parallel-db-before-run

Drop parallel test databases before running create