Add explanation for IPv6 failures

Fix #45: Build Tests not passing

RPC/Address's tests use getaddrinfo to look up IPv6 addresses. The man
page states: "...IPv6 addresses are only returned if the local system
has at least one IPv6 address configured.  The loop‐back address is not
considered for this case as valid as a configured address." So these
tests will only pass on some machines (only guaranteed to work on those
that have at least one IPv6 address configured on an external
interface). Since the tests are still useful, I added a note when they
fail that this might be the reason why.