Add Verify/FailFast CI template

changelogs/unreleased/rspec-rails-fast-failure-template.yml

+---
+title: Add Verify/FailFast CI template
+merge_request: 31812
+author:
+type: added

lib/gitlab/ci/templates/Verify/FailFast.gitlab-ci.yml

+rspec-rails-modified-path-specs:
+  stage: .pre
+  rules:
+    - if: $CI_MERGE_REQUEST_EVENT_TYPE == "merged_result" || $CI_MERGE_REQUEST_EVENT_TYPE == "merge_train"
+      changes: ["**/*.rb"]
+  script:
+    - gem install test_file_finder
+    - spec_files=$(tff $(git diff --name-only "$CI_MERGE_REQUEST_TARGET_BRANCH_SHA..$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA"))
+    - |
+        if [ -n "$spec_files" ]
+        then
+          bundle install
+          bundle exec rspec -- $spec_files
+        else
+          echo "No relevant spec files found by tff"
+          exit 0
+        fi