Tengo algunos códigos como a continuación, quiero escribir pruebas unitarias de mi método. Pero estoy atrapado en métodos asincrónicos. Puedes ayudarme por favor

public class Panel
{
    public int Id { get; set; }
    [Required] public double Latitude { get; set; }
    public double Longitude { get; set; }
    [Required] public string Serial { get; set; }
    public string Brand { get; set; }
}

public class CrossSolarDbContext : DbContext
{
    public CrossSolarDbContext()
    {
    }

    public CrossSolarDbContext(DbContextOptions<CrossSolarDbContext> options) : base(options)
    {
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
    }
}

public interface IGenericRepository<T>
{
    Task<T> GetAsync(string id);

    IQueryable<T> Query();

    Task InsertAsync(T entity);

    Task UpdateAsync(T entity);
}

public abstract class GenericRepository<T> : IGenericRepository<T>
    where T : class, new()
{
    protected CrossSolarDbContext _dbContext { get; set; }

    public async Task<T> GetAsync(string id)
    {
        return await _dbContext.FindAsync<T>(id);
    }

    public IQueryable<T> Query()
    {
        return _dbContext.Set<T>().AsQueryable();
    } 

    public async Task InsertAsync(T entity)
    {
        _dbContext.Set<T>().Add(entity);
        await _dbContext.SaveChangesAsync();
    }

    public async Task UpdateAsync(T entity)
    {
        _dbContext.Entry(entity).State = EntityState.Modified;
        await _dbContext.SaveChangesAsync();
    }
}

public interface IPanelRepository : IGenericRepository<Panel> { }

public class PanelRepository : GenericRepository<Panel>, IPanelRepository
{
    public PanelRepository(CrossSolarDbContext dbContext)
    {
        _dbContext = dbContext;
    }
}

[Route("[controller]")]
public class PanelController : Controller
{
    private readonly IPanelRepository _panelRepository;

    public PanelController(IPanelRepository panelRepository)
    {
        _panelRepository = panelRepository;
    }

    // GET panel/XXXX1111YYYY2222
    [HttpGet("{panelId}")]
    public async Task<IActionResult> Get([FromRoute] string panelId)
    {
        Panel panel = await _panelRepository.Query().FirstOrDefaultAsync(x => x.Serial.Equals(panelId, StringComparison.CurrentCultureIgnoreCase));
        if (panel == null) return NotFound();
        return Ok(panel);
    }
}

public class PanelControllerTests
{
    private readonly PanelController _panelController;
    private static readonly Panel panel = new Panel { Id = 1, Brand = "Areva", Latitude = 12.345678, Longitude = 98.7655432, Serial = "AAAA1111BBBB2222" };

    private readonly IQueryable<Panel> panels = new List<Panel>() { panel }.AsQueryable();
    private readonly Mock<IPanelRepository> _panelRepositoryMock = new Mock<IPanelRepository>();

    public PanelControllerTests()
    {
        _panelRepositoryMock.Setup(x => x.Query()).Returns(panels);
        // I also tried this. I got another error 'Invalid setup on an extension method: x => x.FirstOrDefaultAsync<Panel>(It.IsAny<Expression<Func<Panel, Boolean>>>(), CancellationToken)'
        // _panelRepositoryMock.As<IQueryable<Panel>>().Setup(x => x.FirstOrDefaultAsync(It.IsAny<Expression<Func<Panel, bool>>>(), default(CancellationToken))).ReturnsAsync(panel);
        _panelController = new PanelController(_panelRepositoryMock.Object);
    }

    [Fact]
    public async Task Register_ShouldInsertOneHourElectricity()
    {
        IActionResult result = await _panelController.Get("AAAA1111BBBB2222");
        Assert.NotNull(result);
        var createdResult = result as CreatedResult;
        Assert.NotNull(createdResult);
        Assert.Equal(201, createdResult.StatusCode);
    }
}

Recibo este error

El proveedor de la fuente IQueryable no implementa IAsyncQueryProvider. Solo los proveedores que implementan IEntityQueryProvider pueden usarse para operaciones asincrónicas de Entity Framework.

Creo que necesito burlarme de 'FirstOrDefaultAsync' pero no estoy seguro y no sé cómo hacerlo. Intenté algo, sin embargo, no se pudo compilar.