Migrate to Web Test Runner and fix broken tests

[usergenic]

Added a helpers.js to export a shared wait() function that we can use in tests to wait for renders to complete.

[AndrewJakubowicz]

Should the tests pass in this PR?
When I ran npm i, npm run bootstrap, and then npm run test, I get a test error.

virtualize > uses the provided method to render items
  Error: ResizeObserver loop limit exceeded

Otherwise looking great and only tiny license nits.

[usergenic]

@AndrewJakubowicz thanks for running the tests locally-- I have gotten that error only once since switching so I thought I managed to work out how to await the reflows correctly. Looks like I have to dig in and get a more solid solution.

[graynorton]

@usergenic I don't think this is a timing issue with the tests. The ResizeObserver loop limit error is a genuine but (I believe) benign error that results from virtualizer's specific use of ResizeObserver, so I think what we'll want to do is catch this error and prevent it from causing the tests to fail. See #112 for a bit more background, and we can discuss.

[usergenic]

I added a helper to specifically swallow errors captured by window.onerror which matched the ResizeObserver loop limit exceeded case in test/helpers.js called ignoreBenignErrors. It can be added basically once per file so that it attaches setup/teardown to the Mocha context properly. It still outputs a warning to console so we can see when they happen, but it won't fail tests now.

 🚧 Browser logs:
      Error: IGNORE_ME
        at test/scenarios/testing-setup.test.js:18:19
      Ignored Error: Uncaught Error: IGNORE_ME

test/scenarios/smoke.test.js:

 🚧 Browser logs:
      null
      Ignored Error: ResizeObserver loop limit exceeded

Chrome: |██████████████████████████████| 3/3 test files | 4 passed, 0 failed

[AndrewJakubowicz]

Nit: Some additional tests:

• A test where the thrown error doesn't match the regex and isn't caught.
• Maybe a test that checks that afterEach is cleaning up.

[usergenic]

I'm not quite sure how to use Mocha to test Mocha hooks just yet. But I added a nested case and an explicit error if teardown encounters an unexpected value in window.onerror...

[AndrewJakubowicz]

Sorry for these questions, this is looking great. These are just for context.

Will migrating the screenshot.js tests come in a follow up PR? Currently npm run test:screenshot doesn't work.

Thank you!

[usergenic]

@AndrewJakubowicz yeah I am not deleting screenshot tests yet, but getting them fixed will be a much different undertaking. I tried messing with them but it got really fiddly really fast and there were enough changes in this one.

[AndrewJakubowicz]

LGTM!

[graynorton]

LGTM!