[vstest/subintf] Add vs test to validate sub interface ingress to a vnet (#1642)

What I did
Extended the following vs tests to validate the use case of sub interface ingress linked to a vnet:

sub port interface creation
sub port interface add ipv4 addresses
sub port interface admin status change
sub port interface remove ipv4 addresses
sub port interface removal

Why I did it
Add vs test to validate that #1521 fixes #4238

Author: wendani