meson: don't build classic mesa tests without dri_drivers

Since mesa_classic is build-on-demand the tests will create a demand and
add a bunch of extra compilation.

Fixes: 43a6e849
       ("meson: build mesa test.")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit aaab6242)