[test] Wait for flags to load on regression tests

[m4theushw]

Fixes #5448 (comment)

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	242.1	476.2	298.5	333.44	84.297
Sort 100k rows	ms	471.9	803.6	626.8	658	118.412
Select 100k rows	ms	133.7	225.6	171.5	169.34	33.76
Deselect 100k rows	ms	99.9	180.3	178.3	143.32	31.275

Generated by 🚫 dangerJS against 3e0cd43

[oliviertassinari]

In the project I worked on before MUI, we solved the problem like this:

  await page.evaluate(async () => {
    const selectors = Array.from(document.querySelectorAll('img'))
    await Promise.all(
      selectors.map(img => {
        // The image is already loaded      
        if (img.complete) {
          return null
        }

        return new Promise(resolve => {
          img.addEventListener('load', resolve)
          img.addEventListener('error', resolve)
        })
      })
    )
  })

Is this something that could work? It seems a challenge to keep pathsToNotWaitForFlagCDN in sync with the codebase.