Changes

hnbgw: TC_hnb_disconnected_timeout

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.

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