Visual regression tests > Leverage sharding in CI

With gitlab-org/gitlab-ui!3878 (merged), we are switching to Storybook Test Runner to run visual regression tests. This gives us the ability to parallelize test runs: https://github.com/storybookjs/test-runner?tab=readme-ov-file#4---run-tests-with---shard-flag.

Let's tweak our CI and Test Runner configs to speed up our visual regression tests.