Before this patch, the test tries to remove all children from the 'content'
node except for one, as part of cleaning up. This is unnecessary, because none
of the subtests ever add any additional children to the 'content' element.
(It's also problematic because in late beta & release, 'content' is a special
variable name by default.)
While we're at it, this patch also makes us address the other nodes more
consistently, using the explicit variable declarations at the top of the
script section rather than their implicit ID-granted variable names.
Differential Revision: https://phabricator.services.mozilla.com/D83251
e45bd243c2