Document how we write acceptance tests

[phil-davis]

Update and add to the acceptance test documentation.

This PR is aimed at:

• document the standard grammar forms that we are using to write Given When and Then step text
• give guidelines for how to write the code for new API acceptance test steps
• give guidelines for how to write the code for new CLI acceptance test steps
• give guidelines on what makes a "good" test scenario
• how to write scenarios for bugs Documenting existing behavior when bugs are found QA#601

from issue #156

[settermjd]

On the whole it looks great, just a few points that need addressing.

[mmattel]

I cant comment on the acceptance test content, but the text looks good

[phil-davis]

This was work-in-progress. Anyway, what I had so far was a good start! I will pull master and start a new branch and PR for the next parts.

[mmattel]

👍