retro: create tests for retro pages

Added tests for the known year pages and the 404 case. With theses tests
the retro pages have 100% test coverage

retro/tests.py

@@ -0,0 +1,14 @@
+from django.test import TestCase
+
+from retro.views import RETRO_YEAR_MAP
+
+class RetroTest(TestCase):
+
+    def test_404(self):
+        response = self.client.get('/retro/1999/')
+        self.assertEqual(response.status_code, 404)
+
+    def test_retro(self):
+        for year, _ in RETRO_YEAR_MAP.items():
+            response = self.client.get('/retro/{}/'.format(year))
+            self.assertEqual(response.status_code, 200)