feat: Add single output mode to consolidate all failed Rspec tests in a single file

What does this MR do and why?

This MR enhances the failed_tests.rb script to support generating a single consolidated file containing all failed RSpec tests, instead of only categorizing them by suite (which misses a few tests).

This change is backward compatible and adds the foundation for predictive test metrics functionality.

Output

The output is stored in tmp/latest_failed_tests/rspec_all_failed_tests.txt file

spec/features/achievements/achievements_spec.rb spec/tooling/ci/changed_files_spec.rb

References

Part 1 of the MR split feat: Create a new job to gather metrics for Pr... (!191658 - closed))

Relates to gitlab-org/quality/analytics/team#95 (closed),