Add test coverage for retryable jobs if merge pipeline

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In the MR !42495 (merged) we fixed the bug reported here #244862 (closed). Which allowed users to retry failed jobs that were in a merge train pipeline, which cannot be retired.

We should add test coverage to spec/views/projects/pipelines/show.html.haml_spec.rb with this particular use case to ensure we don't show the retry job icon if jobs are not retryable.