MSC: add a test case to check T3212 expiration during paging

Long story short: some time ago I noticed that OsmoMSC crashes if
T3212 expires during the Paging procedure. This is not the case
anymore (as the test case shows) and apparently the bug has been
fixed, hovewer I believe it makes sense to add this test case.

Change-Id: If9147ae8b07d5120d2853b9acda2313910ac48be
3 files changed