Testing Data Layer - Repository Pattern

I am working on a web api project in asp.net. I have decided to put the data access layer aside and focus on the business layer, but for that i have to find a way to mimic the data access layer.

I am working with repositories, my application is a bit more complicated than that but for now lets say that i have two domain objects, User and Ticket:

class User {
    public int Id { get; set; }
    public string Name { get; set; }

    public IList<Ticket> Tickets { get; set; }
}

class Ticket {
    public int Id { get; set; }
    public int Type { get; set; }
    public string Name { get; set; }
    public User Owner { get; set; }
}

I have the user test repository:

class UserTestRepository : IUserRepository {
    private TestDataContext context;

    public UserTestRepository (TestDataContext Context) {
        this.context = Context;
    }

    public User GetById (int Id) {
        return context.Users.Where(u => u.Id == Id).Single();
    }

    public void Add (User User) {
        context.AddUser(User);
    }
}

And also the ticket test repository which looks the same. The TestDataContext looks something like that:

class TestDataContext {
    public IList<User> Users { get; set; }
    public IList<Ticket> Ticets { get; set; }

    private int userCount;
    private int ticketCount;

    public TestDataContext () {
        userCount = 0;
        ticketCount = 0;

        Users = new List<User>();
        Tickets = new List<Ticket>();

        Seed(); // Not implemented yet...
    }

    public void AddUser (User User) {
        User.Id = userCount;
        userCount++;

        Users.Add(User);
    }

    public void UpdateUser (User User) {
        int index = Users.IndexOf(Users.Where(u => u.Id == User.Id).First());
        Users[index] = User;
    }

    public void DeleteUser (User User) {
        Users.Remove(Users.Where(u => u.Id == User.Id).First());
    }

    // Same three methods for Tickets...
}

Now lets say that i want to add a new Ticket to user with id 3, i would do this:

Ticket ticket = new Ticket () {
    Type = 1,
    Name = "My Ticket",
    Owner = new User () { Id = 3 }
};

TicketsRepository.Add(ticket);

The problem is that when i fetch the user with id 3, i would want to see this ticket that i inserted in the user's Tickets list.

I though of doing something like that:

// This is in TestDataContext

public void AddTicket (Ticket Ticket) {
    Ticket.Id = ticketCount;
    ticketCount++;

    Tickets.Add(Ticket);

    int userIndex = Users.IndexOf(Users.Where(u => u.Id == Ticket.User.Id).First());
    Users[userIndex].Tickets.Add(Ticket);
}

But the same i will have to do for updating and deleting, and for more complicated data relations in would be a really difficult task.

So my question is, am i doing it right? Is this how i supposed to test my data layer?

I don't want to just test data fetching, i want to test inserting and deleting because those i also very important functions of my application.

Thank you