MSC_Tests: add function to check if a subscriber is in VLR

The control interface of osmo-msc is able to return a list with all
active subscribers from the VLR. Lets add a function, so that we can
check from TTCN3 if a specified subscriber is known by the VLR or not.

Change-Id: I7661ae55afe34795c3701d59795331b32d64c988
Related: OS#3614
1 file changed