In order to create a new locally database to run tests you can create your migrations in the test project by taking in care that you can use your repository db context in another VS project.
To achieve this result you can do the following in the Package Manager Console
Enable-Migrations -ContextProjectName MyProject.Repository -ContextTypeName MyProject.Repository.MyDbContext -ProjectName MyProject.Tests -StartUpProjectName MyProject.Tests -ConnectionStringName "MyConnectionString"
Add-Migration "Init Test Db"
Update-Database -ProjectName MyProject.Tests -StartUpProjectName MyProject.Tests -ConnectionStringName "MyConnectionString"