[REG dev] Cannot run GUI tests

1. Clone qt5.git and initialize qtbase:

   git clone git://code.qt.io/qt/qt5.git
   cd qt5/
   perl init-repository --codereview-username rlohning --module-subset=qtbase
   

2. Build Qt.
   I use:

   ../qt5/configure -platform linux-clang -opensource -confirm-license -no-widgets -prefix $PWD/qtbase -release && nice cmake --build .
   

3. Using the resulting Qt build, build a test project, e.g.:

   CXX=clang++ /tmp/qt-bisect/build-qt/qtbase/bin/qt-cmake ~/dev/src/qtbase/tests/auto/gui/text/qfont/  && cmake --build .
   

4. Try running the resulting test binary:
   I get:

   ./tst_qfont
   qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
   This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
   
   Available platform plugins are: minimal, offscreen, linuxfb, minimalegl, vkkhrdisplay, vnc, eglfs, wayland-brcm, wayland-egl, wayland.
   

Doing the same on an earlier revision of the dev branch succeeds.