Changes

msc: add TC_silent_call_{start_stop,active_conn}

This patch adds testcases for the silent-call feature:

* MSC_Tests.TC_silent_call_start_stop
  Test starting and stopping silent call via the VTY.
* MSC_Tests.TC_silent_call_active_conn
  Test initiating a silent call while there's an active connection.
  The MSC is expected to be smart and reject such an attempt.

Change-Id: I3ef22fbb5a05f69cc8aea7f42e05f6e1d6c4a8b6
Related: osmo-msc.git I93913d189800d71f82c013b6e946bd63db362f65
Related: osmo-msc.git I52b3be26c61cadacc4783b5c324809ecd6906b36

testenv: Dockerfile: fix Suites: stable-backports

Using "stable-backports" broke now that Debian 13 is out. Use
"bookworm-backports" instead.

Fix for:
  E: The value 'bookworm-backports' is invalid for APT::Default-Release as such a release is not available in the sources

Change-Id: Icc46b6dc2493806dc12f0ff442abd70fe5b48eb8