PCU_Tests: fix BSSGP in TC_ta_idle_dl_tbf_ass

When we send BSSGP DL UD, we should include an IMSI, since we are
expecting the paging request to appear on the PCH

Related: OS#5927
Change-Id: If62c2c7db9717cd08116374ee6ca939211bdf01e
1 file changed