Failure to run tests under xvfb: X11 connection broke

(I originally saw similar symptoms to QTBUG-45025 when trying to run tests in for example qtsensors with 'xvfb-run -s "-screen 0 1024x768x24" -a'. That is, I got 'QXcbConnection: no screens available' + 'Aborted'. Log http://is.gd/Stcdvs)

After cherry-picking https://codereview.qt-project.org/#/c/108209/ on top of 5.5.0 alpha, the error message changes to:
—
The X11 connection broke: Unsupported extension used (code 2)
XIO: fatal IO error 6 (No such device or address) on X server ":99"
after 6 requests (6 known processed) with 0 events remaining.
—
Log at http://is.gd/wKA2oB

xvfb has been the standard way we have run unit tests on headless builders from Qt 5.0 to 5.4. We've liked using xvfb to have actual rendering done instead of using QT_QPA_PLATFORM=minimal

I further cherry-picked https://codereview.qt-project.org/#/c/109456/ but it didn't have an effect on this problem.