update_ci_reference_files.py: Ignore tests with no AMI tag (not an error)

While dealing with another FT0-Violating MR, I encountered an minor issue with the update_ci_reference_files.py. It aborted with the error

FATAL: Did not find an AMI tag in this test unit-tests. Aborting.

because the CI-output had also a failing unit test (timeout). This is not actually an error, so I turned that condition into non-fatal warning.

cc @emoyse