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.

This cleans up the `test:in_parallel` rake task and uses that for the Jenkins script.