I’m following the guides, where I find the instructions to seed the database before running the tests? Thanks.
The tests when they will run will import the test data in the DB and run tests against it.
Or you can generate test data as well for your own manual tests, see: Generating test data - Introducing the Piwik Platform - Analytics Platform - Matomo
See also more info on running tests PHP Tests: Develop - Matomo Analytics (formerly Piwik Analytics) - Developer Docs - v3