[action] [PR:15109] Fix testQosSaiDscpQueueMapping

[mssonicbld]

Description of PR

Summary: Fixes #277

Type of change

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

Back port request

• [ ] 202012
• [ ] 202205
• [ ] 202305
• [ ] 202311
• [x] 202405

Approach

What is the motivation for this PR?

Regression introduced by #14232 14232

06:34:12 __init__._fixture_generator_decorator L0088 ERROR | 
KeyError(8)
Traceback (most recent call last):
 File "/data/tests/common/plugins/log_section_start/__init__.py", line 84, in _fixture_generator_decorator
 res = next(it)
 File "/data/tests/qos/qos_sai_base.py", line 2455, in tc_to_dscp_count
 for dscp, tc in dscp_to_tc_map.items():
KeyError: 8

How did you do it?

Get rid of assumption of 8TCs from the code.

How did you verify/test it?

Ran the test on Arista 7260X3 platform.

Any platform specific information?

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

Documentation

[mssonicbld]

Original PR: https://github.com/sonic-net/sonic-mgmt/pull/15109