This patch attempts to fix intermittent failures by:
1. using `ensureMaximizedWindow()` where appropriate. While nothing
seemed wrong per se with the previous approach, resizing to the
original height was done to restore the previous window dimensions.
Since the window is maximized before, it makes sense to reuse this
helper function, especially given that it is a bit more robust than
just awaiting an event.
2. not awaiting the `resize` event, which does not seem to be fired all
the time when `resizeBy()` is used. It was mostly fine, except when
it wasn't... Since we have a `.waitForCondition()` call right after,
we can probably skip the await of this event.
Differential Revision: https://phabricator.services.mozilla.com/D169088
d03c7afe84