Normally we start Xvfb as a background task, then run the tests from the
same script. However, in interactive mode we were starting Xvfb, the script
would exit, and then we would potentially run tests later on from another
script. Unforunately this meant that Xvfb was dying with the first script
and tests would fail.
This patch runs Xvfb in a screen session so that it will still be available
later on when running an interactive shell.
MozReview-Commit-ID: EduVyglo2BG
--HG--
extra : rebase_source : 6e2c40ee16f80792f038fd581168e181a2c4bf51