Make running Selenium tests using Sauce Labs easy

Make running Selenium tests using Sauce Labs easy

If SAUCE_USERNAME and SAUCE_ACCESS_KEY environment variables are not
defined, tests are run with `npm run selenium`, as usual.

https://www.mediawiki.org/wiki/Selenium/Node.js/Inside_MediaWiki-Vagrant
https://www.mediawiki.org/wiki/Selenium/Node.js/Target_MediaWiki-Vagrant

If the environment variables are defined, to run tests using Sauce Labs
and targeting Beta Cluster, run this:

```
export SAUCE_USERNAME=***
export SAUCE_ACCESS_KEY=***
export MW_SERVER=https://en.wikipedia.beta.wmflabs.org
./node_modules/.bin/wdio tests/selenium/wdio.conf.js
```

https://www.mediawiki.org/wiki/Selenium/Node.js/Target_beta_cluster_using_Sauce_Labs

Bug: T139740
Change-Id: I622d9c55ae595ab772e733388d209623e3a9f81e