chore(tests): move from args to commands in cli

[LeoMcA]

Would recommend reviewing with whitespace changes hidden

Relates to #1588:

Converting the test cli to use commands, and merging the visual-report cli in: using a long string of options makes the command hard to use, and complicates implementation

I think my original idea was to have a single "test" command to run everything locally, but the existing behaviour wasn't giving that, and made life more complicated, requiring me to have a separate command for generating the visual report which I'll merge into the commands here (which PR that happens on depends on which gets merged first!)

This will also make further follow-ups simpler, like being able to run a single test, and some kind of watch mode.

If we need a "run everything" mode we can always add that as another or the default command.

[github-actions]

86e7aff was deployed to: https://fred-pr1590.review.mdn.allizom.net/