support/testing: add test for xserver/Mesa OpenGL/glxinfo

support/testing: add test for xserver/Mesa OpenGL/glxinfo

This test allow to check if the xserver with GLX is working properly.
This is a basic test but it allow to trigger the current bug reported
by [1].

To test if the glxinfo test is working, you can change "-display :0" by
"-display :1" in the glxinfo command line.

[1] https://bugs.buildroot.org/show_bug.cgi?id=11591

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Youssef Harmouch <youssef.harmouch@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>