Changes

hnbgw: TC_hnb_disconnected_timeout; Osmocom_CTRL_Functions

Test the new timer hnbgw X35: discard all hnb_persistent state after a
given timeout.

While a hnb_persistent is present, its rate_ctrs and stat_items remain
present. When it is discarded, these vanish again. Use this to verify
that discarding and re-registering a hnb_persistent works.

Teach the CTRL functions that retrieve rate counters to return counters
of -1 when a requested object is not present, instead of erroring and
stopping the test. Like this we can expect absence of an object.

Set X35 to five seconds, drop the Iuh link, wait and verify that the
rate counter group for the hnb_persistent is gone.

Then reconnect and disconnect again, to provoke errors like
use-after-free.

Depends: osmo-hnbgw Ic819d7cbc03fb39e98c204b70d016c5170dc6307
Related: SYS#6773
Change-Id: Ibec009203d38f65714561b7c28edbdbd8b34e704