[functions] debug disabled tests and re-enable them if possible

[tmatsuo]

The PR #3774 disabled some tests in functions directory.

• pubsub/main_test.py
• slack/main_test.py

Debug them and re-enable them if possible.

@ace-n Let me know if you can not fix this.

[tmatsuo]

For slack/main_test.py, I think only thing needed is to set an appropriate value for "SLACK_SECRET". I assume this value can be just calculated somehow so that it is not a really secret. With that assumption, I put a placeholder in noxfile_config.py for adding the value.

If the value is really a secret, you have to remove noxfile_config.py and add it to testing/secrets.tar.enc.

[tmatsuo]

fixed by #3808