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/layermap/models.py

@@ -36,11 +36,17 @@ class City(NamedModel):
     dt = models.DateField()
     point = models.PointField()
 
+    class Meta:
+        app_label = 'layermap'
+
 
 class Interstate(NamedModel):
     length = models.DecimalField(max_digits=6, decimal_places=2)
     path = models.LineStringField()
 
+    class Meta:
+        app_label = 'layermap'
+
 
 # Same as `City` above, but for testing model inheritance.
 class CityBase(NamedModel):