test/test_ipaddr: always set ifa_flags

test/test_ipaddr: always set ifa_flags

do not rely on the uninitialized bits on stack for testing, otherwise
there is chance that we have following test failure:

64: ../src/test/test_ipaddr.cc:226: Failure
64: Expected equality of these values:
64:   two.ifa_addr
64:     Which is: 127.0.0.1:57677
64:   result
64:     Which is: 127.0.0.1:57677

the root cause is just that the "result" is actually `one.ifa_addr`,
which is marked UP as that's what the uninitialized bits on stack
indicate.

Signed-off-by: Kefu Chai <kchai@redhat.com>