tests: Fix node warnings
Created by: chrismwendt
Upgrades web-ext
from 4 to 5. 4 depended on a version of git-rev-sync
which had some kind of a cycle with shelljs
.
The only breaking change from web-ext
5 does not impact us AFAICT https://github.com/mozilla/web-ext/releases/tag/5.0.0
Found via node --trace-warnings