Add simple tests against the project using IPv4 and IPv6

CONTRIBUTING.md

@@ -1,5 +1,13 @@
 ## Development Environment FAQ
 
+### How to run tests?
+
+From the project root directory, execute the following command:
+
+```sh
+python ./manage.py test
+```
+
 ### How to maintain `requirements.txt`?
 
 The canonical list of installation requirements (including development requirements) is maintained in the [`setup.py`](setup.py) file.

tests.py

@@ -0,0 +1,22 @@
+from django.urls import reverse
+from django.test import Client, TestCase
+
+
+class UngleichScreeningTaskTests(TestCase):
+    def setUp(self):
+        self.ipv4_client = Client(REMOTE_ADDR="127.0.0.1")
+        self.ipv6_client = Client(REMOTE_ADDR="::1")
+
+    def test_project_using_ipv4(self):
+        url = reverse("index")
+        response = self.ipv4_client.get(url)
+        self.assertContains(
+            response, "Sorry, only reachable by IPv6", status_code=400, html=True
+        )
+
+    def test_project_using_ipv6(self):
+        url = reverse("index")
+        response = self.ipv6_client.get(url)
+        self.assertContains(
+            response, "A friendly cat picture", status_code=200, html=True
+        )