[bug] Handle new tags structure

[shilman]

storybookjs/storybook#26634 introduces a new tag structure where tags can be defined at the project level and the test runner should be updated to handle the new behavior. For index.json mode, nothing should need to be done, but on the source transformation side the test runner probably needs fixing.

Also, we might consider making the test tag implicit. So if no --includeTags/--excludeTags is passed, then only include stories tagged with the test tag (which will be all stories by default in 8.1+)