Revert "Removed unneeded app_label definitions in gis_tests"

This reverts commit e0cc36f615.
The problem is the following:
  * With non-gis backends, gis_tests sub apps are not discovered
    (see runtests.py)
  * Consequently, the app_label is inferred from the gis_tests
    AppConfig
  * Then models with same names in different sub apps conflict
    because of same model_name/app_label pair.

tests/gis_tests/geoadmin/models.py

@@ -10,6 +10,9 @@ class City(models.Model):
 
     objects = models.GeoManager()
 
+    class Meta:
+        app_label = 'geoadmin'
+
     def __str__(self):
         return self.name