How to mock database in testing? Golang

I have a problem. I am writing tests and was able to simulate a database for sql query using go-sqlmock. But how can I test handlers, that is, the headers I can’t imagine. Here is an example:

func (env *EnvironmentUser) GetUserHandler (w http.ResponseWriter, r *http.Request) {
    params := mux.Vars(r)
    id, err := strconv.Atoi(params["id"])
    if err != nil {
        w.WriteHeader(http.StatusNotFound)
        return
    }
    user, err := env.Db.GetUser(int(id))
    if err != nil {
        if err == sql.ErrNoRows {
            w.WriteHeader(http.StatusUnauthorized)
        }
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    json.NewEncoder(w).Encode(user)
    w.WriteHeader(http.StatusOK)
}

GetUser - listing:

row := db.QueryRow("SELECT * FROM users WHERE id = $1", id)
    err = row.Scan(&user.Id, &user.Email, &user.Login, &user.Fullname,
        &user.Password, &user.AccVerified)
    if err != nil {
        return models.User{}, err
    }
    return user, nil

How do I check the correctness of all haders??