drm/tests: shmem: Fix memleak

drm/tests: shmem: Fix memleak

The drm_gem_shmem_test_get_pages_sgt() gets a scatter-gather table using
the drm_gem_shmem_get_sg_table() function and rightfully calls
sg_free_table() on it. However, it's also supposed to kfree() the
returned sg_table, but doesn't.

This leads to a memory leak, reported by kmemleak. Fix it by adding a
kunit action to kfree the sgt when the test ends.

Reported-by: Philipp Stanner <phasta@mailbox.org>
Closes: https://lore.kernel.org/dri-devel/a7655158a6367ac46194d57f4b7433ef0772a73e.camel@mailbox.org/
Fixes: 93032ae634d4 ("drm/test: add a test suite for GEM objects backed by shmem")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250408140758.1831333-1-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>