Verify BFD traffic egress queue and default ToS

[kperumalbfn]

Description of PR

Verify BFD traffic egressing out of Queue 7.
Verify BFD traffic from DUT with default ToS 192.

Summary:
Fixes # (issue)
When the BFD state is up, check queue counters to validate BFD packets using queue 7.
Modified bfd_responder to check the received packet's ToS value.

Type of change

improvement

Approach

What is the motivation for this PR?

To verify egress queue of BFD traffic and default ToS of BFD packets.

How did you do it?

Use DUT's 'show queue counter ' and check for non-zero packets in queue 7.
Check captured BFD packet's ToS value.

[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.................................................Passed
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/bfd/test_bfd.py:19:1: E302 expected 2 blank lines, found 1
tests/bfd/test_bfd.py:310:1: E302 expected 2 blank lines, found 1
tests/bfd/test_bfd.py:338:1: E302 expected 2 blank lines, found 1
tests/bfd/test_bfd.py:343:7: E111 indentation is not a multiple of 4
tests/bfd/test_bfd.py:344:7: E111 indentation is not a multiple of 4
tests/bfd/test_bfd.py:345:7: E111 indentation is not a multiple of 4
tests/bfd/test_bfd.py:352:1: E302 expected 2 blank lines, found 1
tests/bfd/test_bfd.py:442:80: E128 continuation line under-indented for visual indent
tests/bfd/test_bfd.py:443:80: E128 continuation line under-indented for visual indent

flake8...............................................(no files to check)Skipped
...
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[prabhataravind]

lgtm

[siqbal1986]

looks good