One Database Per Developer

Testing can be simplified if you can get your database in a known state before a test is run. A database should only be used for one test at a time; otherwise the database state cannot be guarantied.

So multiple developers working on the same project should have their own database instance to prevent data corruption. This also simplifies database cleanup, as you don't necessarily need to revert it to its initial state (See GoodSetupNoCleanup).