Fix UserGroupForm + tests for this form.

- Non-K-Fêt group membership is no longer erased by the account edit
  form.
- Add some tests to ensure proposed choices in this form corresponds to
  K-Fêt groups + test case for #161.

Fixes #161

kfet/tests/test_forms.py

@@ -0,0 +1,48 @@
+# -*- coding: utf-8 -*-
+
+from django.test import TestCase
+from django.contrib.auth.models import User, Group
+
+from kfet.forms import UserGroupForm
+
+
+class UserGroupFormTests(TestCase):
+    """Test suite for UserGroupForm."""
+
+    def setUp(self):
+        # create user
+        self.user = User.objects.create(username="foo", password="foo")
+
+        # create some K-Fêt groups
+        prefix_name = "K-Fêt "
+        names = ["Group 1", "Group 2", "Group 3"]
+        self.kfet_groups = [
+            Group.objects.create(name=prefix_name+name)
+            for name in names
+        ]
+
+        # create a non-K-Fêt group
+        self.other_group = Group.objects.create(name="Other group")
+
+    def test_choices(self):
+        """Only K-Fêt groups are selectable."""
+        form = UserGroupForm(instance=self.user)
+        groups_field = form.fields['groups']
+        self.assertEqual(len(groups_field.choices), len(self.kfet_groups))
+
+    def test_keep_others(self):
+        """User stays in its non-K-Fêt groups."""
+        user = self.user
+
+        # add user to a non-K-Fêt group
+        user.groups.add(self.other_group)
+
+        # add user to some K-Fêt groups through UserGroupForm
+        data = {
+            'groups': [group.pk for group in self.kfet_groups],
+        }
+        form = UserGroupForm(data, instance=user)
+
+        form.is_valid()
+        form.save()
+        self.assertEqual(len(user.groups.all()), 1+len(self.kfet_groups))