feat(core): add Ruby group to TEST_PATTERN_GROUPS

Ruby ecosystems split between two test frameworks with different
file-naming conventions:

  RSpec       — `*_spec.rb`, canonically under `spec/`
                (discourse, homebrew, gitlab, fastlane, rubocop)
  Minitest    — `*_test.rb` / `test_*.rb`, canonically under `test/`
                (rails/rails, activerecord, ruby/ruby)

Both top-level dirs are now caught by EXACT_DIR_NAMES (`spec` was
added in the previous commit), but individual files sometimes leak
elsewhere — engine test files under `app/**/*_test.rb` in Rails
engines, gem repos that colocate small `*_spec.rb` beside
`lib/**/*.rb`. The file globs are what catch that shape.

Tests: assert Ruby sub-header and all three globs; extend the
stable-order invariant to place Ruby after Rust.