Ensure that merge trains contain a specific list of CI jobs

The list of CI jobs run in "merge train" is static and we should ensure
that it only changes intentionally. Otherwise we could slow down the
merge train by accident (which we did in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/172742).

spec/dot_gitlab_ci/ci_configuration_validation/shared_context_and_examples.rb

@@ -122,13 +122,18 @@
 
 RSpec.shared_examples 'merge train pipeline' do
   let(:ci_merge_request_event_type) { 'merge_train' }
+  let(:expected_job_names) do
+    %w[
+      dont-interrupt-me
+      pre-merge-checks
+    ]
+  end
 
   it "succeeds with expected job" do
     expect(pipeline.yaml_errors).to be_nil
     expect(pipeline.errors).to be_empty
     expect(pipeline.status).to eq('created')
-    expect(jobs).to include('pre-merge-checks')
-    expect(jobs).not_to include('upload-frontend-fixtures')
+    expect(jobs).to eq(expected_job_names)
   end
 end