Update `database.yml.sample`: add separate test db

[aaronskiba]

Fixes #3396

Changes proposed in this PR:

• Prior to this change, config/database.yml was configured such that running tests would be executed against the dev db. This would result in the dev db being destroyed after tests. This change ensures that the tests are executed against a separate test db and that the dev db persists while testing.
• NOTE: config/database.yml is set via the cp 'config/database.yml.sample', 'config/database.yml' step when bin/setup postgresql is executed.