mochitests: only include errors in the logs
Migrate mochitest of Arthur Edelstein to mozmill for QA.
mochitest: skip the test if no failure file was created
xpcshell: skip test if no xunit file was created
Add a TBBTestSuite::XServer module

This will be used to start an X server with a dummy driver, and a window
manager, to run the tests in.

The Xdummy script and the xorg.conf files have been taken from xpra.org.
BrowserUnitTests.pm: don't use xvfb if the option is not set
Start an X server and window manager

Add an --no-xdummy option to avoid doing this.

We are now starting an X server with the dummy driver, and a window
manager, to run the tests, instead of xvfb.

Some mochitests are failing when they are running inside xvfb instead of
a real window manager.
browser-unit: increase testsuite_version and tag