[get_interface_addr6] Test mocking & goto exit

Implement mocking out of get_interface_addr6 for testing, and use a
goto: cleanup_exit pattern of returning, to simplify the code flow.
[or/nodelist] Make a few local functions static.
[test_addr] Use mocking in get_stable_addr6 test

Mock out get_interface_address6 within tests of
test_get_stable_interface_address6