Update docs about how to test with hubot

[joeyguerra]

Remove hubot-test-helper as the recommendation.

Situation

hubot-test-helper doesn't support the latest Hubot version. The last commit was 6 years ago (March 12, 2018). There are 2 open PRs that are trying to update it to newer versions of Hubot.

Options

1. Submit a PR, updating the code to utilize the latest version of Hubot
2. Add a DummyAdapter and Example test to the Hubot codebase and the generator as examples
3. Remove the testing section of the docs

Thoughts

I can work on 1 and 2 over time. Option 2 won't take very long. Option 2 will take more time, thought, effort. Not impossible, but requires motivation. I do think providing some guidance on how to test scripts would be beneficial. So I don't want to option 3 at all.