Failing dialog-modal_datepicker.js test

[howard-e]

A failure I've confirmed to not be flaky, first observed in #2936:

error shown in build: Element query returned no results: #example [role="dialog"] table.dates td[data-date="2019-01-01"]

not ok 144 - dialog-modal_datepicker › content/patterns/dialog-modal/examples/datepicker-dialog.html [data-test-id="gridcell-button-aria-selected"]: aria-selected on selected date
  ---
    name: AssertionError
    message: >-
      Element query returned no results: #example [role="dialog"] table.dates
      td[data-date="2019-01-01"]
    assertion: fail
    at: |-
      Object.queryElements (test/util/queryElements.js:16:7)
      async assertAttributeValues (test/util/assertAttributeValues.js:17:18)
      async test/tests/dialog-modal_datepicker.js:238:5
  ...

I'm unsure why this happens at the moment but raising the issue because it doesn't seem to be unique to #2936. That PR also doesn't affect files which can cause that. The test is currently at test/tests/dialog-modal_datepicker.js#L228:L266

I'm also able to consistently replicate this failure on main by running the following command locally:

npm run regression -- -t test/tests/dialog-modal_datepicker.js

[howard-e]

I haven't been able to reproduce or observe in newer PRs this issue happening since the ~week it was reported. Changes may have happened in the infrastructure around that time which helped prevent this from happening. Will reopen if it pops up once again

cc @ccanash