[storage_backend] Add testcase to test sub port stability

[lolyu]

Description of PR

Summary:
Fixes #3835

Type of change

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

Back port request

• 201911

Approach

What is the motivation for this PR?

Test sub port stability if some other sub ports are removed.

How did you do it?

Add test_routing_between_sub_ports_unaffected_by_sub_ports_removal to cover this scenario that it will verify the testing sub port still routes packets if some other sub ports are removed.

How did you verify/test it?

sub_port_interfaces/test_sub_port_interfaces.py::TestSubPorts::test_routing_between_sub_ports_unaffected_by_sub_ports_removal[same-TCP-UDP-ICMP-PORT] PASSED                                                                                                           [ 50%]
sub_port_interfaces/test_sub_port_interfaces.py::TestSubPorts::test_routing_between_sub_ports_unaffected_by_sub_ports_removal[different-TCP-UDP-ICMP-PORT] PASSED                                                                                                      [100%]

========================================================================================================================= 2 passed in 521.30 seconds =========================================================================================================================

Any platform specific information?

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

Documentation

[lgtm-com]

This pull request introduces 1 alert when merging 3d3fc65 into 4b157b8 - view on LGTM.com

new alerts:

• 1 for Nested loops with same variable

[lolyu]

/AZP run

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