refactor(test_db): use sqlx::test instead

[juleswritescode]

While working on the completions for roles, I had unexpected test failures.

The reason was that roles are setup at the postgres-instance level, and while databases are unique to each test, all tests share the same instance.

To solve this problem, we'll use test migrations – there, we can set up a couple of roles, and use them throughout the test. Sqlx provides the Migrator for this.

We can then use the sqlx::test macro for whenever we need a PgPool in a test – it'll create a unique DB, run the migration files, and best of all drop the database once the test finishes.

So no more of these dang volumes:

[psteinroe]

That's a really nice feature of sqlx!