Our <select multiple> has enough padding so that both sides take more space that
one list item, and since this test sets the height explicitly and <select> has
box-sizing: border-box by default, page-up / page-down steps through one less
item than what the test expects.
I think this is a test bug, but let me know if you want some bug on file to
consider reducing this padding for non-native theme.
Differential Revision: https://phabricator.services.mozilla.com/D66239
--HG--
extra : moz-landing-system : lando
