Skip test_buffer.py for cisco-8000 platforms.

[rraghav-cisco]

All tests in qos/test_buffer.py check for dynamic buffer model and then procede. Since cisco-8000 platform supports only traditional model, this does nothing, and spends a lot of time unnecessarily. This PR is to skip them at the entry level itself for cisco-8000 platforms.

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

Back port request

• 201911
• 202012
• 202205

Approach

To save time running on cisco-8000 platforms.

How did you do it?

Added skip condition in common skip condition yaml file.

How did you verify/test it?

Ran it with cisco platform:
--------------------------------------------------------------------------------------------------- live log sessionfinish ---------------------------------------------------------------------------------------------------
20:17:06 init.pytest_terminal_summary L0064 INFO | Can not get Allure report URL. Please check logs
================================================================================================== short test summary info ===================================================================================================
SKIPPED [1] qos/test_buffer.py:1991: These tests don't apply to cisco 8000 platforms, since they support only traditional model.
SKIPPED [16] qos/test_buffer.py: These tests don't apply to cisco 8000 platforms, since they support only traditional model.
================================================================================================= 17 skipped in 2.34 seconds =================================================================================================
vxr@02b4831febde:/data/tests$

Without this fix, we get 3 minutes to skip all:
<testsuite errors="0" failures="0" name="pytest" skipped="17" tests="17" time="164.315">

Any platform specific information?

Specific to cisco-8000 platforms only.

[rraghav-cisco]

@kevinwangsk , pls check.