Unit test address/get_if_addrs6_list_no_internal should succeed if there are only internal addresses

Split off legacy/trac#19960 (moved):

It is ok for the log message generated by address/get_if_addrs6_list_no_internal to be either:

• "connect() failed", or
• "unable to create socket", or
• "Address that we determined via UDP socket magic is unsuitable for public comms."

But we don't currently allow the third option.

address/get_if_addrs6_list_no_internal: [forking] 
  FAIL src/test/test_address.c:850: expected log to contain "connect() failed" o
r "unable to create socket"  Captured logs:
    1. err: "Address that we determined via UDP socket magic is unsuitable for p
ublic comms.\n"

  [get_if_addrs6_list_no_internal FAILED]