Update slider and settings tests following the changes from #20264 and #19417
We need to update the slider and settings tests to take into account the changes from legacy/trac#20264 (moved), as well as legacy/trac#19417 (moved) on the alpha/nightly channel.
The slider tests need to be adapted to expect different values depending on the Tor Browser version being tested (similar to what we do in the settings tests).