[1.5.x] Merged pagination tests

It is simpler/cleaner to have all pagination tests in a single file.
Refs #16122.
Backport of 45802e1248 from master.

tests/regressiontests/pagination/models.py

@@ -0,0 +1,11 @@
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+@python_2_unicode_compatible
+class Article(models.Model):
+    headline = models.CharField(max_length=100, default='Default headline')
+    pub_date = models.DateTimeField()
+
+    def __str__(self):
+        return self.headline