tools/testing/cxl: Simulate auto-assembly failure

tools/testing/cxl: Simulate auto-assembly failure

Add a cxl_test module option to skip setting up one of the members of the
default auto-assembled region.

This simulates a device failing between firmware setup and OS boot, or
region configuration interrupted by an event like kexec.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260327052821.440749-9-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>