MAINTAINERS: Add tools/testing/cxl files to CXL

MAINTAINERS: Add tools/testing/cxl files to CXL

tools/testing/cxl contains the unit test infrastructure for mocking CXL
hierarchies. These are under the purview of the CXL subsystem maintainers.
Add the 'F:' entry for this to MAINTAINERS so that get_maintainer.pl
works as expected for patches to this area.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/20231026-vv-mainteners-fix-v1-1-0a0f25634073@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>