tools/testing/cxl: Mock one level of switches

The CXL port enumeration process adds intermediate CXL ports that are
discovered between "root" CXL ports enumerated by 'cxl_acpi' and
endpoints enumerated by 'cxl_pci + cxl_mem'. Test the dynamic discovery
of intermediate switch ports in a CXL topology.

Link: https://lore.kernel.org/r/164298432189.3018233.13142151550113000967.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>