limit the modification of pg_num to only occur on the specific platform to fix testQosSaiHeadroomPoolWatermark failure

[XuChen-MSFT]

Description of PR

Summary:
Fixes # (issue)

Type of change

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

Back port request

• 201911
• 202012
• 202205
• 202305

Approach

What is the motivation for this PR?

different input qos parameter for testQosSaiHeadroomPoolSize and

wrong qos paramreter for testQosSaiHeadroomPoolWatermark,

"/root/env-python3/bin/ptf","--test-dir","saitests/py3","sai_qos_tests.HdrmPoolSizeTest",
... ... omitted ... ...
pgs_num=46;
pgs=[3, 4]",

... ... omitted ... .... 
[(0, 3, 5), (0, 4, 6), (1, 3, 5), (1, 4, 6), (2, 3, 5), (2, 4, 6), (3, 3, 5), (3, 4, 6), (4, 3, 5), (4, 4, 6), (5, 3, 5), (5, 4, 6), (6, 3, 5), (6, 4, 6), (7, 3, 5), (7, 4, 6), (8, 3, 5), (8, 4, 6), (9, 3, 5), (9, 4, 6), (10, 3, 5), (10, 4, 6), (11, 3, 5), (11, 4, 6), (12, 3, 5), (12, 4, 6), (13, 3, 5), (13, 4, 6), (14, 3, 5), (14, 4, 6), (15, 3, 5), (15, 4, 6), (16, 3, 5), (16, 4, 6), (17, 3, 5), (17, 4, 6), (18, 3, 5), (18, 4, 6), (19, 3, 5), (19, 4, 6), (20, 3, 5), (20, 4, 6), (21, 3, 5), (21, 4, 6), (22, 3, 5), (22, 4, 6)]       >>>>> and then it will test 46 couple of parameters

RCA:
for this case pgs_num value should be 4.
but here, its value is 46, and then it will test consume share buffer using 46 couple of parameters.
finally, caused test failure

the reason is that pg_num was increated in PR #8213 before ptf test.

How did you do it?

there is conditional statement to limit the modification of pg_num to only occur on the specific platform in similar test case "testQosSaiHeadroomPoolSize".
so, limit the modification of pg_num for testQosSaiHeadroomPoolWatermark as well

How did you verify/test it?

pass local test

Any platform specific information?

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

Documentation

[mssonicbld]

Cherry-pick PR to 202305: #10376

[mssonicbld]

Cherry-pick PR to 202205: #10377