Use `dropDatabase` mongoose helper
Use `dropDatabase` mongoose helper

On archlinux w/ mongodb v3.4.1 and mongoose v4.6.4, I ran into
github.com/Automattic/mongoose/issues/4634 when running tests

This change uses the `dropDatabase` mongoose helper instead of
bypassing mongoose via `db.dropDatabase`

cised-ca/social-enterprise-directory,chimo/social-enterprise-directory