[Test Gap][Macsec and PFC][<T2>] Add test to verify PFC Clear functionality

[abdosi]

Test Gap Description

Below need to to be tested on macsec tested. Currently we can not run qos testmodule on macsec testbed/pipeline
So we need pointed test case for upstream LC.

1. Send PFC frames unencrypted on macsec link ==> This one we can check with Vendor if there is any way to trigger PFC using debug cli command. [without disabling TX as that will need syncd rpc docker dependency when running macsec test case]
2. Expect UnEnncrypted frame on macsec link. ==> This should be straightforward . We can leverage pfc_gen script on Fanout to send PFC Frame and see DUT PFC counter increment

Test Plan

No response

Test environment

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Attach files (if any)

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[abdosi]

@rlhui and @judyjoseph for viz.