Improve test progress denominator in Testing Module

[JReinhold]

The current progress indicator in the Testing Module uses Vitest’s completion status X / Y, where the denominator steadily increases as the test run completes. Vitest has to do it this way because it cannot statically analyze all the test files.

But CSF is statically analyzable and Storybook knows exactly how many tests are going to be run. So we should be able to provide an accurate numerator for the run.

This should both work when testing all stories, and with focused tests, and we need to be mindful of the 'test' tag on stories.