Generate PFC storm using PFC backpressure on Arista DCS-7060X6 devices

[veronica-arista]

Description of PR

Add a pfc_gen script for Arista DCS-7060X6 fanout devices to generate pfc pause frames by setting PFC backpressure status in hardware. Enable the storm generation to use this script on interfaces that are connected to Arista DCS-7060X6 fanout devices running EOS.

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 202012
• 202205
• 202305
• 202311
• 202405

Approach

What is the motivation for this PR?

Adds more reliable PFC storm generation for interfaces connected to Arista DCS-7060X6 fanout devices

How did you do it?

How did you verify/test it?

Ran the pfcwd tests on t1 switch with interfaces connected to Arista DCS-7060X6 device, ensured the new script was used and pfc storm was appropriately detected.

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[mssonicbld]

The pre-commit check detected issues in the files touched by this pull request.
The pre-commit check is a mandatory check, please fix detected issues.

Detailed pre-commit check results:
trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing tests/common/helpers/pfc_gen_th5.py

fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/common/helpers/pfc_gen_th5.py:7:1: F401 'binascii' imported but unused
tests/common/helpers/pfc_gen_th5.py:59:31: F541 f-string is missing placeholders
tests/common/helpers/pfc_gen_th5.py:62:52: W605 invalid escape sequence 'S'
tests/common/helpers/pfc_gen_th5.py:62:67: W605 invalid escape sequence '['
tests/common/helpers/pfc_gen_th5.py:62:75: W605 invalid escape sequence 'd'
...
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