TODO: glx: meson: build dri based glx tests, only with -Dglx=dri

The library itself (libGL) is only built when -Dglx=dri, yet it's
accompanying tests are build even with -Dglx=xlib.

Adjust the guards, so we don't build the tests when they are not
applicable

v2:
 - Reword commit message (Dylan)
 - Drop build_by_default hunk (Dylan)

Fixes: a47c525f ("meson: build glx")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 9527f9ea)