drm/tests: reduce drm_mm_test stack usage

The check_reserve_boundaries function uses a lot of kernel stack,
and it gets inlined by clang, which makes __drm_test_mm_reserve
use even more of it, to the point of hitting the warning limit:

drivers/gpu/drm/tests/drm_mm_test.c:344:12: error: stack frame size (1048) exceeds limit (1024) in '__drm_test_mm_reserve' [-Werror,-Wframe-larger-than]

When building with gcc, this does not happen, but the structleak
plugin can similarly increase the stack usage and needs to be
disabled, as we do for all other kunit users.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20221215163511.266214-1-arnd@kernel.org