Composite projects in SSDT are a really useful way to logically split databases up while still being able to deploy into a database as a whole. Even if you have a fairly simple database in a single SSDT project you should put your unit tests somewhere. What I like to do is have my solution configured like:

Main.dbproj - actual production code and reference data Main.UnitTests.dbproj - unit test code + references to tSQLt etc The unit tests have a “Same Database” reference pointing to the main project so it is as if my tests were in the same project.