class Number(models.Model):
    _name = "bn"
    _description = 'Number'
    _inherit = ['mail.thread']
    _rec_name = 'business_no'
    bno = fields.Char(string='Bnumber', required=True)
    mno_id = fields.Many2one('bn.mno', string='MNO', required=True)
    account_id = fields.Many2one('account.account', string='Account', required=True)
    service_id = fields.Many2one('account.account', string='Service', required=True)
    _sql_constraints = [
        ('bno', 'unique(bno)', 'Bnumber already exists!')
    ]

In this model their are 4 mendotry fields.I want to write all possible test cases for this senerion.
Please tell me best approach for handling possible cases:
1) All fields provided with correct value.
2) All fields provided with incorrect value.
3) All fields not provided.

Please tell me good approach for writing test case?