[Bug] Test-runner misses stories in symlinked directories in normal mode

[shilman]

Describe the bug

Storybook's new CI process uses symlinked directories to link in stories to "sandbox" projects.

Storybook follows the symlinks, so it produces an index.json that contains all the stories. Running the test runner in --index-json mode catches them.

However running the test runner in "normal" mode does not.

Steps to reproduce the behavior

In the SB repo prior to merging #19255:

yarn task --task test-runner --template cra/default-js