This fixes the two `#addExposure()` calls in `UrlbarView.#updateResults()` so
that hidden exposures are properly counted. Right now we overcount them because
we record them in cases where the result would not have been shown anyway.
The new test tries to cover all possible `UrlbarView.#updateResults()` paths:
* When the view is full and a new result can't replace any of the old rows, the
new result is appended and hidden. If the query is then canceled, the new row
will never be shown
* When the vew isn't full, a new row can be appended and visible immediately
* When a new row replaces an old row, it can be visible immediately
Differential Revision: https://phabricator.services.mozilla.com/D199176
0ec8c8bac6