Fix race conditions / timeouts in app test

[mccalluc]

We could up the timeout... but that will mean a stall when the element really isn't there. This might also reflect a real usability problem: Users could stack up actions and the app would seem to be unresponsive.

• Maybe we should lock controls until the last action is resolved?
• Or maybe there is a way for higher priority actions to override lower priority?

       # -- Download results --
       page.get_by_role("button", name="Download results").click()
       expect_not_visible(pick_dataset_text)
>       expect_not_visible(perform_analysis_text)

Looking at the trace, it looks like it might have gotten bogged down in recalculating the >graphs and didn't respond to the mouseclick to move to the next page quickly enough. >Is there a risk of the application being unresponsive?

Originally posted by @mccalluc in #103 (comment)