Add a Testid kebabcase cop

What does this MR do and why?

As per gitlab-org/quality/quality-engineering/team-tasks#1965 (comment 1493947286) , as part of unifying the page test selectors, we want to add a cop that checks that our QA page object element items are using snake-case strings.

How to set up and validate locally

Run rubocop, no errors should be seen

Try and add element :foo to a QA page, rubocop should throw message. Try and add element 'foo_bar', rubocop should throw message.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.