public async Task Given_GetGradeById_When_IdIsValid_Then_OkStatusCode()
{
//Arrange
var guid = new Guid("ef7e98df-26ed-4b21-b874-c3a2815d18ac");
mockRepo
.Setup(g => g.FindById(guid))
.ReturnsAsync(new GradeDetailsModel());
// Act
var result = await controller.GetGradeById(guid);
// Assert
Assert.IsType <OkObjectResult>(result);
}