tests: Install latest chromium-headless from updates-testing
tests: Install latest chromium-headless from updates-testing

Current chromium 61 has a bug in `Network.setCookie()` which is hard to
work around and fixed in 62. Install chromium-headless from
updates-testing, which gets current upstream versions much more timely.

Closes #162 
Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>
cockpit-project/cockpituous,cockpit-project/cockpituous