Normalize test file paths to project root in RSpec formatter

Problem Statement

The RSpec formatter stores test file paths relative to the RSpec execution directory rather than the GitLab project root. For QA tests, RSpec runs from ./qa/, so paths are stored as qa/specs/... instead of qa/qa/specs/....

This creates issues when constructing GitLab file URLs in analytics dashboards, requiring conditional logic to prepend qa/ for QA specs.

Goal

Store all test file paths relative to the GitLab project root to enable straightforward URL construction.

Technical Considerations

The without_relative_path method in lib/gitlab_quality/test_tooling/test_metrics_exporter/test_metrics.rb currently only strips the ./ prefix. It should normalize paths to the project root by prepending the appropriate prefix based on the execution context.