Correctly handle folder arg in parallel_runner

What does this MR do and why?

Fix parallel runner incorrectly forcing single process when default single folder argument is used.

Closes [E2E] Fix parallel runner with default arguments (gitlab-org/quality/analytics/team#13 - closed)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Reduce tier-2 and tier-3 pipeline execution time (&16180 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.