[FIX] account: add a test on settings change

Because of f206714a and subsequent commits, adding groups may raise.
We ensure that switching between tax included and tax excluded keeps working.

Closes #37481

closes odoo/odoo#37633

Forward-port-of: #37517
Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/account/tests/__init__.py

@@ -14,6 +14,7 @@ from . import test_reconciliation_widget
 from . import test_payment
 from . import test_reconciliation
 from . import test_search
+from . import test_settings
 from . import test_tax
 from . import test_invoice_taxes
 from . import test_templates_consistency

addons/account/tests/test_settings.py

+from odoo.addons.account.tests.account_test_classes import AccountingTestCase
+from odoo.tests import tagged
+
+
+@tagged('post_install', '-at_install')
+class TestSettings(AccountingTestCase):
+
+    def test_switch_taxB2B_taxB2C(self):
+        """
+        Since having users both in the tax B2B and tax B2C groups raise,
+        modifications of the settings must be done in the right order;
+        otherwise it is impossible to change the settings.
+        """
+        # at each setting change, all users should be removed from one group and added to the other
+        # so picking an arbitrary witness should be equivalent to checking that everything worked.
+        config = self.env['res.config.settings'].create({})
+
+        config.show_line_subtotals_tax_selection = "tax_excluded"
+        config._onchange_sale_tax()
+        config.flush()
+        config.execute()
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_excluded'), True)
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_included'), False)
+
+        config.show_line_subtotals_tax_selection = "tax_included"
+        config._onchange_sale_tax()
+        config.flush()
+        config.execute()
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_excluded'), False)
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_included'), True)
+
+        config.show_line_subtotals_tax_selection = "tax_excluded"
+        config._onchange_sale_tax()
+        config.flush()
+        config.execute()
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_excluded'), True)
+        self.assertEqual(self.env.user.has_group('account.group_show_line_subtotals_tax_included'), False)