Fix model validation tests to rely on full_clean()

The obj.clean() method on django models is enforced on obj.full_clean() not
obj.save()

coin/members/tests.py

@@ -363,10 +363,14 @@ class MemberTests(TestCase):
         (prenom, nom) ou pseudo ou nom d'organization
         """
         member = Member(username='blop')
-        self.assertRaises(Exception, member.save)
+        with self.assertRaises(Exception):
+            member.full_clean()
+            member.save()
+
+        with self.assertRaises(Exception):
+            member.full_clean()
+            member.save()
 
-        member = Member()
-        self.assertRaises(Exception, member.save)
 
 
 class MemberAdminTests(TestCase):