MAN-3694 -- Add a waitUntilSettled function that waits for an element to finish updating

[mpharoah-d2l]

https://desire2learn.atlassian.net/browse/MAN-3694

This PR adds a public waitUntilSettled function that more reliably waits for an element to finish updating. Our team intends to use this in multiple repos to help reduce vdiff flake.

[dlockhart]

This feels like too much D2L-specific knowledge leakage into what we're hoping to keep as a bare bones testing library. Have you looked at the getLoadingComplete stuff (or mixin) to handle this kind of thing? Each component that does async calls would need to be aware of it, but that's the way we'd always imagined this stuff working.

If you did want to try and write a single thing that "waits for things to settle", you could still use getLoadingComplete in a shared mixin and put this logic there.

Alternately, you could create your own fixture method that calls into @brightspace-ui/testing's fixture (which calls into the open-wc one) but then additionally waits for more stuff?

[mpharoah-d2l]

Alternately, you could create your own fixture method that calls into @brightspace-ui/testing's fixture (which calls into the open-wc one) but then additionally waits for more stuff?

We actually do that in d2l-rubric, but also add the waitUntilSettled method to the fixture so we can call it again after simulating clicks and such. We were hoping to add this to the testing repo to avoid copy-pasting the same code to our other repos. Is there another library we can put this in if its too D2L specific for this one?

[dlockhart]

Hmm I'm not sure... there's a lot of Siren-specific stuff in here so maybe as a testing utility in siren-sdk? Or does your team have a shared utils library?

[mpharoah-d2l]

No, but I guess we can just make one

[mpharoah-d2l]

Going to just copy paste it in d2l-outcomes instead