How to implement database seeding for testing with Entity Framework

There's alot to assume from your question: the way I understand it, is you would pass the relevant DbContext to your seeder class anyway. Assuming by 'testing', you mean just putting appropriate test data into a SQL database, then a .NET Core Class Library where your seed class exists is enough; it just needs to know about the DbContext.

Example seeder class:

using System.Linq;
using System.Threading.Tasks;

using BusinessLogic.Entities;  //this is where your business/EF entities live

namespace DataAccessLayer.Data.Services
{
    public class SeederService
    {
        #region FIELDS
        private readonly TestContext _passedInDbContext;

        #endregion

        #region CONSTRUCTOR
        public SeederService(TestContext passedInDbContext)
        {
            _passedInDbContext = passedInDbContext;
        }

        #endregion

        #region METHODS PUBLIC
        public async Task<bool> SeedDatabaseAsync()
        {
            if (!_passedInDbContext.Currencies.Any()) //see if any entries already exist in the Currency SQL table
            {
                //test data
                _passedInDbContext.Currencies.AddRange
                    (
                        new Currency("GBP", "GBP United Kingdom Pounds", "£", 1.000m),
                        new Currency("USD", "USD United States Dollar", "$", 1.200m),
                        new Currency("EUR", "EUR Euro", "€", 1.100m)
                    );
            }

            await _passedInDbContext.SaveChangesAsync();

            return true;
        }
        #endregion
    }
}

Hope this helps somewhat. Of course, you would want to set up a test SQL database and migrate/update/seed this database with the appropriate connection string.