Tests: Download Chromium for e2e and JS tests only.

This reduces the chance of random failures due to network timeouts and avoids the overhead of unnecessarily downloading ~120 MB for PHP tests.

Follow-up to [47449], [47459], [47867], [48177].

Fixes #50649.
Built from https://develop.svn.wordpress.org/trunk@48458


git-svn-id: https://core.svn.wordpress.org/trunk@48227 1a063a9b-81f0-0310-95a4-ce76da25c4cd