library/library_checkout/tests/test_checkout_mass_message.py

from odoo.tests.common import TransactionCase

class TestWizard(TransactionCase):
    def setUp(self, *args, **kwargs):
        super(TestWizard, self).setUp(*args, **kwargs)
        # Add test setup code here...

        # Setup test data
        admin_user = self.env.ref('base.user_admin')
        self.Checkout = self.env['library.checkout'].with_user(admin_user)
        self.Wizard = self.env['library.checkout.massmessage'].with_user(admin_user)

        a_member = self.env['library.member'].create({'name': 'John'})
        self.checkout0 = self.Checkout.create({
            'member_id': a_member.id})

    def test_button_send(self):
        """Send button should create messages on Checkouts"""
        # Add test code