Microsoft.Data.Tools.UnitTest NuGet Package

Microsoft.Data.Tools.UnitTest.dll is a database unit test component in SQL Server Data Tools (SSDT).
SSDT transforms database development by introducing a ubiquitous, declarative model that spans all the phases of database development inside Visual Studio.

You can use SSDT Transact-SQL design capabilities to build, debug, maintain, and refactor databases. You can work with a database project, or directly with a connected database instance on or off-premise.
You can use SQL Server unit tests to establish a baseline state for your database and then to verify any subsequent changes that you make to database objects.
SSDT team's blog - https://blogs.msdn.microsoft.com/ssdt/
SQL Server Data Tools page - https://docs.microsoft.com/sql/ssdt/sql-server-data-tools
More details about SQL Server unit Tests - https://docs.microsoft.com/sql/ssdt/verifying-database-code-by-using-sql-server-unit-tests.