MLBEDSW-10589: Fix the "NPU only" tensor condition in graph packing

An "NPU only" tensor means a tensor that has only NPU producers and
only NPU consumers. The logic was wrong as it allowed CPU consumers,
this lead to some CPU tensors being added as placeholders and
missing from the resulting graph.

Also changed the supported ops unit test to silence log output.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: I511d7bb2419c17dc4230eb791a0a84cd3ce7dedb