[1.7.x] Fixed #22402 -- Consolidated model_inheritance tests.

The model_inheritance_same_model_name tests couldn't be run without the
model_inheritance tests. Make the problem go away by merging them.

Thanks timo for the report.

Backport of 0097d38 from master

tests/model_inheritance/same_model_name/models.py

@@ -0,0 +1,19 @@
+"""
+
+Model inheritance across apps can result in models with the same name,
+requiring an %(app_label)s format string. This app tests this feature by
+redefining the Copy model from model_inheritance/models.py.
+"""
+
+from django.db import models
+
+from model_inheritance.models import NamedURL
+from django.utils.encoding import python_2_unicode_compatible
+
+
+@python_2_unicode_compatible
+class Copy(NamedURL):
+    content = models.TextField()
+
+    def __str__(self):
+        return self.content