Fix cron_parser_spec.rb (`master` branch failure on Nov 5th)

What does this MR do?

Pipelines on master branch are getting failure because of spec/lib/gitlab/ci/cron_parser_spec.rb.

This is because the spec is ambiguous for Regular time vs DST(Daylight saving time). We should elaborate it to prevent edge case failure.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Close #39817 (closed)