tests: properly select renderer for DRM-backend

tests: properly select renderer for DRM-backend

The test suite is dealing only with headless-backend tests.
In order to make it able to run DRM-backend tests, we have
to properly select the renderer that it will use.

This patch add the command line option --use-pixman if the test
defines the DRM-backend renderer as RENDERER_PIXMAN, and it will
add nothing to the command line if it defines RENDERER_GL (the
DRM-backend default renderer is already GL). Also, if the user
defines the DRM-backend renderer as RENDERER_NOOP, the test will
fail (as it should, since DRM-backend does not implement it).

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>