drm/test: add a test suite for GEM objects backed by shmem

This patch introduces an initial KUnit test suite for GEM objects
backed by shmem buffers.

Suggested-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Marco Pagani <marpagan@redhat.com>

v5:
- using __drm_kunit_helper_alloc_drm_device() to avoid local struct
v4:
- Add missing MMU dependency for DRM_GEM_SHMEM_HELPER (kernel test robot)
v3:
- Explicitly cast pointers in the helpers
- Removed unused pointer to parent dev in struct fake_dev
- Test entries reordering in Kconfig and Makefile sent as a separate patch
v2:
- Improved description of test cases
- Cleaner error handling using KUnit actions
- Alphabetical order in Kconfig and Makefile

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130171417.74162-1-marpagan@redhat.com