Add support for bot-invoked slow tests (#672)

Author: shadeMe

.github/workflows/explosionbot.yml

@@ -23,5 +23,5 @@ jobs:
         env:
           INPUT_TOKEN: ${{ secrets.EXPLOSIONBOT_TOKEN }}
           INPUT_BK_TOKEN: ${{ secrets.BUILDKITE_SECRET }}
-          ENABLED_COMMANDS: "test_gpu"
+          ENABLED_COMMANDS: "test_gpu,test_slow"
           ALLOWED_TEAMS: "spacy-maintainers"