I found this Post since I had the Need to Mock a DbSet<X>:

https://sinairv.github.io/blog/2015/10/04/mock-entity-framework-dbset-with-nsubstitute/

Basicaly to Mock a DbSet Using Substitute you can do the following

IQueryable<X> samples = new List<X>{...}.AsQueryable();

DbSet<X> DbSetMock = Substitute.For<DbSet<X>, IQueryable<X>>();
((IQueryable<X>)mockSet).Provider.Returns(samples.Provider);
((IQueryable<X>)mockSet).Expression.Returns(samples.Expression);
((IQueryable<X>)mockSet).ElementType.Returns(samples.ElementType);
((IQueryable<X>)mockSet).GetEnumerator().Returns(samples.GetEnumerator());

IDbContext databaseMock = Substitute.For<IDbContext>();
databaseMock.X = mockSet;