[action] [PR:14860] fix for test_snmp_numpsu failure

[mssonicbld]

Description of PR

Summary: test_snmp_numpsu TC fails because of change in stdout output, observed in 202405 & 202311.

Fixes # (issue)
stdout_lines will be use to fetch the numpsus value, if the stdout_lines is empty / the value can't be typecasted; then in all scenarios TC wouldn't break, instead it will Error out gracefully. Sample stdout_lines considered -

1. ["PSU0: Topology teardown completed successfully upon PSU removal", "2"]
2. ["2"]
3. []

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?

Observed test_snmp_psu failing in 202405 & 202311 images due to change in the stdout structure.

How did you do it?

Earlier stdout output was simply getting typecasted, instead now it will use stdout_lines & check its length, if it has a non-zero length, then it will pick the last element & try typecasting it; numpsus count if exists, will always be the last element in the output.

How did you verify/test it?

Run changes with 202405 & 202311 images on a T0 setup and all of them passed.

[mssonicbld]

Original PR: #14860