Stray chromedriver processes after running scripts/run-module-bundlers-smoketests.js

[Rob--W]

Tested on Linux using 4712d92
STR:

1. cd test/ && run-browsers-smoketests.sh
2. Look at the process list, e.g. pgrep chromedriver or ps | grep chromedriver

Expected: No stray chromedriver processes.
Actual: After each run, there are four additional stray chromedriver processes that haven't been killed:

/path/to/webextension-polyfill/node_modules/chromedriver/lib/chromedriver/chromedriver --port=[random port here]

[Rob--W]

The .sh script was replaced with a JS script in #167 , but the issue is still present.