[Instant] speed up test instant-validation suite (#90214)
This PR speeds up the `instant-validation` test suite from 450s down to
~100s. We were wasting a lot of time in `waitForNoErrorToast()`, which
ends up waiting almost 15s by default.
The trick is to add some debug logs inside app-render to track when
instant validation started/ended. If we know that the validation is
finished, we can just wait for a short time, because if there were
errors, a toast would be displayed very quickly.
Note that is a stopgap -- when we add a "Validating..." indicator, we
won't have to emit the log and can just wait for the indicator instead.
But in the meantime, waiting for the tests is pretty painful, so this is
worth doing.
