test: split conn_update() into update attach-flags and update policy

test: split conn_update() into update attach-flags and update policy

Since ordinary connections are only interested in the attach-flags and
policy holders in policies, split conn_update() into:
1) conn_update_attach_flags()
2) conn_update_policy()

This way we use the conn_update_policy() function in test-kdbus-policy
with a policy-holding connection and we pass all the tests. This
prevents messing up with the attach-flags.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>