Revert "Removed unneeded app_label definitions in gis_tests"

This reverts commit e0cc36f6.
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.