fix tcpdump issue in bgp update timer testing

[jcaiMR]

Description of PR

Fix test_bgp_update_timer.py issue on T0 and T1, which caused by background running tcpdump issue.

Summary:
Fixes # (issue)

Type of change

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

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

Fix test_bgp_update_timer.py issue on T0 and T1, which caused by background running tcpdump.
In script using "nohup <> &" running without waiting for tcpdump tool real running on DUT.
So in some failure case, bgp update sendout but tcpdump still not ready to capture packets and cause failure with saying bgp update not found.

How did you do it?

Wait until tcpdump tool running on DUT before continue testing

How did you verify/test it?

Keep running 10 times on KVM environment without failure.

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.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/bgp/test_bgp_update_timer.py:45:1: E302 expected 2 blank lines, found 1
tests/bgp/test_bgp_update_timer.py:51:1: E302 expected 2 blank lines, found 1

flake8...............................................(no files to check)Skipped
check conditional mark sort..........................(no files to check)Skipped

To run the pre-commit checks locally, you can follow below steps:

1. Ensure that default python is python3. In sonic-mgmt docker container, default python is python2. You can run
   the check by activating the python3 virtual environment in sonic-mgmt docker container or outside of sonic-mgmt
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2. Ensure that the pre-commit package is installed:

sudo pip install pre-commit

3. Go to repository root folder
4. Install the pre-commit hooks:

pre-commit install

5. Use pre-commit to check staged file:

pre-commit

6. Alternatively, you can check committed files using:

pre-commit run --from-ref <commit_id> --to-ref <commit_id>

[StormLiangMS]

LGTM

[yejianquan]

LGTM

[mssonicbld]

Cherry-pick PR to 202305: #9471

[mssonicbld]

@jcaiMR PR conflicts with 202012 branch

[mssonicbld]

Cherry-pick PR to 202205: #9472