cxl/test: Address missing MODULE_DESCRIPTION warnings for cxl_test

cxl/test: Address missing MODULE_DESCRIPTION warnings for cxl_test

Add MODULE_DESCRIPTION() to address the following warnings:
WARNING: modpost: missing MODULE_DESCRIPTION() in test/cxl_test.o
WARNING: modpost: missing MODULE_DESCRIPTION() in test/cxl_mock.o
WARNING: modpost: missing MODULE_DESCRIPTION() in test/cxl_mock_mem.o

[dj: s/CXL test/cxl_test:/ per djbw's comment]

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250429235953.4175408-1-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>