drm/ttm/tests: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_device_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_pool_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_resource_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_tt_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_bo_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_bo_validate_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_mock_manager.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/ttm/tests/ttm_kunit_helpers.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624-md-drivers-gpu-drm-ttm-tests-v2-1-76bb765e19c9@quicinc.com


Signed-off-by: Christian König <christian.koenig@amd.com>