Add integration test environment to Medusa backend

[vholik]

Currently, Medusa only supports a unit test environment, limiting our ability to thoroughly test security and database integration, especially in scenarios where Medusa is extended as a marketplace. To address this, I suggest adding an integration test environment to enhance testing capabilities.

[chrismacp]

Some more information on setting up testing environments within Medusa would be an awesome addition to the docs.

I kept coming across Docker references and test utilities like in this test database helper only to see they were all removed. What really impresses me and improves my ability to get up to speed and work quickly with a framework is the good docs around setting up your environment which I feel is lacking here to some degree. A default set up for a new project would be even better, like the starter project seems to have had before.