support/testing: test_glxinfo: switch to Gallium swrast

Since the mesa3d bump to version 21.0.3 [1], the
BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST option is not supported anymore
since the mesa DRI swrast driver has been removed upstream

So, switch to Gallium swrast.

[1]15a2f9b8

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>