gbproxy: Fix TC_dl_unitdata test failure introduced by 6ee0126971

This commit used a send template (ts) to match a received BSSGP PDU which
doesn't work due to some differences in the length field.
Use tr_BSSGP_IMSI again and change the template restrictions so it
compiles.

Fixes: 6ee0126971c83fd60658761263af559a482f13d9
Change-Id: I85676e96f8d32a9d2c7deadc1d66707b6b8697d0
2 files changed