Codify the requestAnimationFrame delay in test-wait.js as the test-completion timing

[gsnedders]

Rendered

[jcscottiii]

I talked to Xianzhu about this. They said:

I agree with https://issues.chromium.org/issues/41311503#comment3 that the WebDriver should be able to reliably capture an image reflecting the document's latest status, without the client needing to do anything.

I don't remember many details, but crrev.com/c/4505400 now looks like a temporary solution to make the two web test runners do the same thing. It was probably a workaround for the issue where some ref tests had [double-]rAF etc., which required waiting, but the tests didn't declare reftest-wait, causing flaky premature finishes (e.g. crbug.com/40899923, crbug.com/ crrev.com/c/40901018). Now I think we should find and fix the tests instead of working around them in the test runner.