[1.5.x] Fixed #19645 -- Added tests for TransactionMiddleware

Backpatch of f556df90be. Backpatching
these tests so that it will be easier to backpatch the fix for #19707.

tests/regressiontests/middleware/models.py

@@ -1 +1,13 @@
-# models.py file for tests to run.
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+@python_2_unicode_compatible
+class Band(models.Model):
+    name = models.CharField(max_length=100)
+
+    class Meta:
+        ordering = ('name',)
+
+    def __str__(self):
+        return self.name