[test_bgp_stress_link_flap] case hung sometimes due to memory exhaust

[lipxu]

Description of PR

Summary:
Fixes # (issue)
28852952
fix for #14076

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?

The case is flaky
Sometimes the case runs a long time and no response. Especially on kvm device.
Based on current log, it should be related to the memory resource limitation.
The case would create so many threads to flap the neighbor, it would cause kvm device memory exhaust, and same for low memory physical device.
Based on available logs, no obvious memory leak issue.

How did you do it?

The case is for stress link flap, it creates thread per interface to flap.
1: enlarge the delay time for kvm
2: only test one interface for kvm
3: Use one thread to flap all the interfaces for fanout.
4: correct neighbor host
6: add event stop and timeout for thread function to ensure thread exit

How did you verify/test it?

run the case locally and verified using elastictest
https://elastictest.org/scheduler/testplan/66c69c4008761ba27f76ed5d
https://elastictest.org/scheduler/testplan/66c69c2708761ba27f76ed5b
https://elastictest.org/scheduler/testplan/66cdafd1bd14ce56b2e820f7

Any platform specific information?

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

Documentation

[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.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing tests/bgp/test_bgp_stress_link_flap.py

fix end of files.........................................................Passed
check yaml...............................................................Passed
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/bgp/test_bgp_stress_link_flap.py:110:121: E501 line too long (125 > 120 characters)
tests/bgp/test_bgp_stress_link_flap.py:157:1: E303 too many blank lines (3)
tests/bgp/test_bgp_stress_link_flap.py:225:5: E303 too many blank lines (2)
tests/bgp/test_bgp_stress_link_flap.py:234:5: E303 too many blank lines (2)
tests/bgp/test_bgp_stress_link_flap.py:249:5: E303 too many blank lines (2)
...
[truncated extra lines, please run pre-commit locally to view full check results]

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[mssonicbld]

Cherry-pick PR to 202405: #14378