chore(web): determine cause of WebKit-testing concurrency issues + resolve if possible

From our `@web/test-runner` testing configurations...

https://github.com/keymanapp/keyman/blob/e65ba7ca044f633838ed59b315353f90fe8b5889/web/src/test/auto/dom/web-test-runner.config.mjs#L15-L21

Things are quite stable with that `concurrency: 1` in place but rapidly become less-than-stable when it is removed. I never determined the actual cause when I converted our tests over to the new engine.

Similar tidbits also exist in the test configurations for the following, for the exact same reason:
- common/predictive-text
- common/web/gesture-recognizer
- common/web/keyboard-processor
- common/web/lm-worker
- web/src/test/auto/integrated

Labeled a "chore" as this only affects automated testing and we already have the `concurrency: 1` workaround in place. The tests would likely be a bit faster if we could drop the `concurrency` setting, of course.