Let's wrap the test in `act` to get rid of the warning. In practice
(while testing in the browser) the actual warning doesn't seem to affect
the user experience at all.
The `act` function is typed in a strange way (`Promise<undefined> &
void`). Yet the actual contents of the `act` callback is returned as
expected. Therefore we overrode the type of `act` to make sure this
reflects reality better. (Thanks @thecrypticace!)
Also added an additional check to make sure the actual `container` is
available to extra ensure we are not lying by overriding the type.
Robin Malfait
d1ca3a9797