ctrl: subscriber-list-active: list only attached subscribers

I would have liked to add a regression test to verify this, but currently there
is no easy way to run CTRL tests and at the same time have access to the
osmo-msc in a way that simulates an attached subscriber.

Related: OS#2285
Change-Id: I003542b208ecf3713e9e67712d84ccb4c61af14e
1 file changed