ipv6 tests excluded on macos

[adam-thorpe]

Original PR: #1390

[smlambert]

Reminder of the issue, Mac machines on AzDo do not have ipv6 so ipv6 tests running there fail.

There are 2 alternate options under discussion to address this:

• update the upstream tests to check if IPv6 is available/enabled and only run in that case
• add a somewhat generic checkCapability mechanism in TKG to be able to skip tests given certain missing capabilities (including but not limited to ipv6)

[smlambert]

#1529 has re-excluded these tests on Mac OS
but leaving this open to consider the options

[karianna]

I personally feel we should only use infra that supports IPv6 here.