chore(tests): Enable testing on node.js v18.x on GitHub CI (#6619)

* chore(tests): Enable testing on node.js v18.x on GitHub CI

* chore(tests): Remove outdated calls to test:prepare npm script

The calls to `npm run test:prepare` in `tests/scripts/setup_*_env.sh`
should have been removed along with that npm script in PR #4906.

* chore(deps): Fix alphabetisation in package.json

tests/scripts/setup_linux_env.sh

@@ -4,5 +4,4 @@ if [ "${RUNNER_OS}" == "Linux" ]
   then
     Xvfb :99 &
     export DISPLAY=:99 &
-    npm run test:prepare > /dev/null &
 fi

tests/scripts/setup_osx_env.sh

@@ -3,5 +3,4 @@
 if [ "${RUNNER_OS}" == "macOS" ]
   then
     export CHROME_BIN="/Applications/Google Chrome.app"
-    npm run test:prepare > /dev/null &
 fi