[almarklein]

• Fix flaky CI test
• Allow overriding call_later impl for qt and wx
  • asyncio, qt, wx all have Loop._use_call_later_via_thread which default to True, except for asyncio on Pyodide.
  • I first thought I'd use it to fix the test, but was not necessary.
  • Leaving in place even though we don't use it, so we have a lever to pull when problems do arise.

Re-ran CI like 5 times and no test broke.