gtk3 a11y: Force running a11y tests with X11 GDK backend

On Debian testing, tunning the gtk3 a11y tests with

make CppunitTest_vcl_gtk3_a11y

in a Wayland session resulted in a visible
Writer window popping up (while that should not be
the case and isn't when run in an X11 session instead,
as the application is meant to run in a virtual
X server environment via xvfb-run)
and a test failed like this:

.../vcl/qa/cppunit/a11y/atspi2/atspi2.cxx:432:Test1::TestBody
equality assertion failed
- Expected: ATSPI_ROLE_PANEL
- Actual  : ATSPI_ROLE_FILLER

warn:sw.core:15950:15950:sw/source/core/attr/calbck.cxx:196: a 16SwTextFormatColl client added as listener to a 16SwTextFormatColl during client iteration.
warn:sw.core:15950:15950:sw/source/core/attr/calbck.cxx:196: a 16SwLineNumberInfo client added as listener to a 12SwCharFormat during client iteration.
Test1::TestBody finished in: 56011ms
atspi2.cxx:432:Assertion
Test name: Test1::TestBody
equality assertion failed
- Expected: ATSPI_ROLE_PANEL
- Actual  : ATSPI_ROLE_FILLER

Failures !!!
Run: 1   Failure total: 1   Failures: 1   Errors: 0

Force use of the X11 GDK backend by setting env
var `GDK_BACKEND=x11`, which makes that scenario
work as expected.

Change-Id: I7de03593998ffa3bc18d487cb92e00ae75427598
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/156086
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

0 files changed, 0 insertions, 0 deletions