drm/vkms: Compile all tests with CONFIG_DRM_VKMS_KUNIT_TEST

drm/vkms: Compile all tests with CONFIG_DRM_VKMS_KUNIT_TEST

The Kconfig option `CONFIG_DRM_VKMS_KUNIT_TESTS` does not exist. However,
the VKMS format tests use such an option for compilation, meaning that
they are not compiled at all.

Use the Kconfig option `CONFIG_DRM_VKMS_KUNIT_TEST` to compile all VKMS
KUnit tests.

Fixes: 3e897853debd ("drm/vkms: Create KUnit tests for YUV conversions")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://lore.kernel.org/r/20250611205704.334527-1-mcanal@igalia.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>