Hello I am  a little bit confused about unit tests in Odoo 

I have this two models : 

###### Model 1
class VatTaxBreak(models.Model):
    _name = "numidoo_invoicing.vattaxbreak"

    order_id = fields.Many2one('sale.order', string="order_id")
    date = fields.Date(string="Date", required=True)
    currency_id = fields.Many2one(related='order_id.currency_id', depends=['order_id.currency_id'], store=True, string='Currency', readonly=True)
    amount = fields.Monetary(string='Amount', store=True, required=True)
    ref = fields.Char(string="Reference", required=True)

    scanned_doc = fields.Binary(string="Scanned document", required=True)
    scanned_doc_name=fields.Char()

    @api.constrains('date')
    def _date_valid(self):
        for record in self:
            if record.date > datetime.today():
                raise ValidationError('La date de la franchise ne peut pas être dans le future')

    @api.constrains('amount')
    def _amount_valid(self):
        for record in self:
            if record.amount == 0 or record.amount < 0:
                raise ValidationError('Le montant ne peut pas être nul ou négatif')


    @api.constrains('scanned_doc_name')
    def _scanned_doc_name_valid(self):
        for record in self:
            if record.scanned_doc:
                if not record.scanned_doc_name:
                    raise ValidationError('pas de fichier')
                else:
                    # check the file's extension
                    tmp = record.scanned_doc_name.split('.')
                    ext = tmp[len(tmp)-1]
                    if ext != 'pdf' and ext != 'png' and ext != 'jpg':
                        raise exceptions.ValidationError("Le fichier doit être une image (extension png ou jpg) ou bien un fichier pdf")

 
######### Model 2
class SaleOrderNumidoo(models.Model):
    _inherit = 'sale.order'

    vattaxbreak_ids = fields.One2many('numidoo_invoicing.vattaxbreak', 'order_id', string="Child franchise")
    amount_vattaxbreak_total = fields.Monetary(string="Vat tax break", store=True, readonly=True, compute="_amount_vattaxbreak")
    payment_mode = fields.Selection([('especes', 'Espèces'),
                                     ('par_cheque', 'Par chèque'),
                                     ('virement_bancaire', 'Virement Bancaire'),
                                     ('versement', 'Versement')
                                     ('a_terme', 'A terme')])

    @api.depends('vattaxbreak_ids.amount')
    def _amount_vattaxbreak(self):
        """
        Compute the total amounts of the SO.
        """
        for order in self:
            amount_total = 0
            for line in order.vattaxbreak_ids:
                amount_total += line.amount
            order.update({
                'amount_vattaxbreak_total': - amount_total,
            })

    @api.depends('order_line.price_total','vattaxbreak_ids.amount')
    def _amount_all(self):
        super(SaleOrderNumidoo, self)._amount_all()
        for order in self:
            order.update({
               'amount_total': order.amount_total + order.amount_vattaxbreak_total,
            })

And I have three questions : 

- When I create an instance in the class test, should I give the binary file in the Model 1 (vattaxbreak) because it is required?? if yes how would I do that 

- My second question is how to create an instance should I use self.env or unittest.mock?? and why.

- In Model2 (SaleOrderNumidoo) I have a one2many field and in the Model 1 (vattaxbreak) I have many2one field, how would I populate this fields in my instance of tests?