[dv/kmac] cycle-accurate model bugfixes - pt 1
[dv/kmac] cycle-accurate model bugfixes - pt 1

this PR contains several fixes for the KMAC cycle accurate model:

- most are related to KDF transactions, in particular backpressure
  scenarios where the msgfifo fills up while KDF transactions are still
  being continually sent to the DUT.

- fixed a potential race condition where `fifo_empty` could be sampled
  on the same timestep that it is updated internally

Signed-off-by: Udi Jonnalagadda <790cd677b6a22a419ffcfe0eb4293d8866eb70c9@google.com>

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