[Test Gap] A VLAN interface should stay up when all of its member ports are operationally down

[mramezani95]

Description of PR

Summary:
Fixes #14790
When all member ports of a VLAN are down, the VLAN interface should still remain operationally up. The added test file (test_vlan_ports_down.py) performs the following steps:

Setup: Brings down all member ports of a VLAN interface.

1. Asserts that the VLAN interface is operationally Up.
2. Asserts that the VLAN interface's IPv4 and IPv6 subnets are advertised to the T1 neighbors.
3. Asserts that IPv4 decapsulation feature works for the VLAN interface.

Tear-down: Starts up all member ports of the VLAN interface.

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?

When all member ports of a VLAN are down, the VLAN interface should still remain operationally up.

How did you do it?

We select a VLAN and then bring all of its member ports down. Then we assert all 3 conditions mentioned in the summary.

How did you verify/test it?

Ran the test on a virtual T0 switch (with Broadcom ASIC).

Any platform specific information?

N/A

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

T0

[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...............................................................Passed
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/vlan/test_vlan_ports_down.py:17:1: E302 expected 2 blank lines, found 1
tests/vlan/test_vlan_ports_down.py:32:18: E261 at least two spaces before inline comment
tests/vlan/test_vlan_ports_down.py:37:1: E302 expected 2 blank lines, found 1
tests/vlan/test_vlan_ports_down.py:51:1: E302 expected 2 blank lines, found 1
tests/vlan/test_vlan_ports_down.py:75:9: E722 do not use bare 'except'
tests/vlan/test_vlan_ports_down.py:83:9: E722 do not use bare 'except'

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

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