Refactor pipeline editor tabs spec with new validation tab

What does this MR do and why?

Describe in detail what your merge request does and why.

• Changed spec name from pipeline_editor_lint_spec.rb to pipeline_editor_tabs_spec.rb since this name seems to make more sense.
• We introduced a new tab Validate behind FF simulate_pipeline (to replace Lint tab eventually), so I create scenarios for both when FF is enabled vs disabled. Most of the code are redundant now but they will be removed when the flag is removed so I think it's okay not to spend too much time on shrinking them.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.