Add job to check for ee ruby code in foss

This job runs in gitlab MR pipelines when
the pipeline is a pipeline for merged result.

It runs as if foss, without ee/ directory.
It detects ruby foss files changed and runs matching
rspec test files to detect any EE dependency
that might exist in FOSS.

.gitignore

@@ -92,3 +92,4 @@ webpack-dev-server.json
 /.nvimrc
 .solargraph.yml
 apollo.config.js
+matching_foss_tests.txt

.gitlab/ci/rails.gitlab-ci.yml

@@ -324,3 +324,26 @@ db:rollback geo:
     - bundle exec rake geo:db:migrate
 # EE: default refs (MRs, master, schedules) jobs #
 ##################################################
+
+##################################################
+# EE: merged result MR pipelines
+rspec verify-no-ee-in-foss:
+  extends:
+    - .rspec-base
+    - .as-if-foss
+    - .rails:rules:ee-only
+    - .use-pg11
+  script:
+    - git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+    - git fetch origin $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME
+    - run_timed_command "scripts/gitaly-test-build"
+    - run_timed_command "scripts/gitaly-test-spawn"
+    - source scripts/rspec_helpers.sh
+    - scripts/find_foss_tests matching_foss_tests.txt
+    - rspec_simple_job "--tag ~quarantine --tag ~geo --tag ~level:migration $(cat matching_foss_tests.txt)"
+  artifacts:
+    expire_in: 7d
+    paths:
+      - matching_foss_tests.txt
+# EE: merged result MR pipelines
+##################################################

scripts/find_foss_tests

+#!/usr/bin/env ruby
+
+require_relative '../lib/gitlab/popen'
+require_relative '../lib/quality/test_file_finder'
+
+output_file = ARGV.shift
+
+source = ENV.fetch('CI_MERGE_REQUEST_SOURCE_BRANCH_NAME')
+target = ENV.fetch('CI_MERGE_REQUEST_TARGET_BRANCH_NAME')
+
+git_diff = Gitlab::Popen.popen_with_detail(%w[git diff --name-only] + ["origin/#{target}...origin/#{source}"])
+
+unless git_diff.status.success?
+  warn "**** #{git_diff.cmd.join(' ')} had the following error(s):"
+  warn git_diff.stderr
+  exit git_diff.status.exitstatus
+end
+
+changed_files = git_diff.stdout.split("\n")
+
+tests_to_run = changed_files.flat_map do |file|
+  test_files = Quality::TestFileFinder.new(file, foss_test_only: true).test_files
+  test_files.select { |f| File.exist?(f) }
+end
+
+File.write(output_file, tests_to_run.uniq.join(' '))