Sync stable-2.5 test infrastructure with devel: - shippable.yml - test/runner/ - test/sanity/ - test/utils/ - docs/docsite/rst/dev_guide/testing/sanity/ - test/integration/targets/*/aliases
* Move sanity into directory. * Omit abstract classes from returned subclass list. * Split sanity tests out into plugins. * Fix abstract class handling for Python 3.