tests/gl_basic: add support for GBM platform
tests/gl_basic: add support for GBM platform

This way we can also run waffle's tests through headless CI ;-)

Fixes #34: https://github.com/waffle-gl/waffle/issues/34
Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@intel.com>

waffle-gl/waffle,chadversary/waffle,waffle-gl/waffle,waffle-gl/waffle,dcbaker/waffle,chadversary/waffle,chadversary/waffle,dcbaker/waffle,waffle-gl/waffle,dcbaker/waffle