[code-infra] Optimize regression tests

[Janpot]

• Remove the need for 1300+ links to be rendered in the DOM while running regression tests
• Make it possible to scope a single test in running test:regressions. Just add route = ['<path>'] at https://github.com/Janpot/material-ui/blob/632de3d7de97eadb3cc225aca5a3e3a4ba25fb10/test/regressions/index.test.js#L51
• Remove the links during running the tests (only render them once on initial render to determine which tests to run)

Results:

• On my local, about ~15s decrease in runtime (~11%)
• Gained possibility to scope one or more tests in the production run, very helpful for debugging [code-infra] Fix flaky dashboard screenshot #43890

[mui-bot]

Netlify deploy preview

https://deploy-preview-43889--material-ui.netlify.app/

Bundle size report

No bundle size changes (Toolpad)
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Generated by 🚫 dangerJS against 632de3d

[JCQuintas]

Should this be documented somewhere?

Make it possible to scope a single test in running test:regressions. Just add route = [''] at https://github.com/Janpot/material-ui/blob/632de3d7de97eadb3cc225aca5a3e3a4ba25fb10/test/regressions/index.test.js#L51

[Janpot]

Should this be documented somewhere?

I think we can document it if we add specific API for it. The "feature" is more a side-effect of the changes I did to optimize them.