When developing run all the tests once with phpunit if you are just getting started. This will setup the test database for you in mongodb, as there is a test listener class which runs the migrations for a new database named jikan-test. (by default, feel free to override it if required)

When writing tests you should not use DatabaseMigration and DatabaseTransaction traits.

If the search index requires resetting between tests, use ScoutFlush trait.
If the database contents requires resetting between tests, use SyntheticMongoDbTransaction trait.

ScoutFlush: Runs the scout:flush artisan command.

SyntheticMongoDbTransaction: empties the mongodb database between tests, by gathering all app models, and dropping all items in them.

Rationale

Jikan API uses mongodb. We think that the laravel ecosystem is not very well tailored for mongodb in terms of unit tests, or at least that's our experience so far. If you try to use DatabaseMigration trait in tests, you would run the migration for each test, which takes around 5-6 seconds on a somewhat powerful machine. The next sensible choice would be DatabaseTransaction trait, however that would require the contributors to set up a mongodb replicaset locally (1)(2). The middle ground is to run migrations once during the test lifecycle, and during local development it is enough to run the migrations once. This significantly reduces the time required to run the tests.