public async Task AverageAsync_Student_CalculatesAverageCorrectly()
{
using var context = new UniversityContext(builder.Options);
var repository = new GradeRepository(context);
// From UniversityContext OnModelCreating
var initialNumber = 3;
var count = await repository.AverageAsync(grade => grade.StudentId == 3, grade => grade.Mark);
Assert.That(count, Is.EqualTo(initialNumber));
}