Use the headless Ruby gem in the automated test suite

In our test suite the headless Ruby gem could replace our xvfb and video capturing stuff in run_test_suite. Moving that code “into Ruby” would give us more flexibility. For instance:

• We would only run the xvfb server when we need it, i.e. only for @product features, and not for @source features where it’s just weird.
  * We’d probably get better cleaning up of the leftover sockets in /tmp/.X11-unix/ (#8707 (closed)).
• We could capture per-scenario videos (and perhaps delete all but failing the ones?).

It’s also a step towards removing the run_test_suite shell wrapper to have the complete test suite in Ruby only.

Unfortunately it’s not packaged in Debian, but before we try making that happen we should investigate whether this gem truly suites our needs, so it’s not a wasted effort.

Related issues

• Related to #8707 (closed)
• Related to #10148 (closed)

Original created by @anonym on 8947 (Redmine)