tools/testing/cxl: Slow down the mock firmware transfer

The cxl-cli unit test for firmware update does operations like starting
an asynchronous firmware update, making sure it is in progress, and
attempting to cancel it. In some cases, such as with no or minimal
dynamic debugging turned on, the firmware update completes too quickly,
not allowing the test to have a chance to verify it was in progress.
This caused a failure of the signature:

  expected fw_update_in_progress:true
  test/cxl-update-firmware.sh: failed at line 88

Fix this by adding a delay (~1.5 - 2 ms) to each firmware transfer
request handled by the mocked interface.

Reported-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/20231026-vv-fw_upd_test_fix-v2-1-5282fd193883@intel.com


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