BSC_Tests: ramdomize IMSI in f_gen_test_hdlr_pars()

Using the same IMSI in each test case makes it harder to correlate
subscriber connections in the SUT with particular test cases.  A
good example is a problem described in OS#5337: some test cases
create dangling subscriber entries that never got released:

  OsmoBSC# show subscriber all
   IMSI             TMSI      Use
   001019876543210  ffffffff  3 (3*paging-start)
   001010000100001  ffffffff  1 (conn)

With this patch I am getting the following output:

  OsmoBSC# show subscriber all
   IMSI             TMSI      Use
   001016017428234  ffffffff  1 (paging-start)
   001014631230680  ffffffff  1 (paging-start)
   001011161500382  ffffffff  1 (paging-start)
   001010000100001  ffffffff  1 (conn)

so I was able to correlate the following test cases:

* (001016017428234) TC_lcs_loc_req_for_active_ms_ta_req,
* (001014631230680) TC_lcs_loc_req_for_active_ms_le_timeout2,
* (001011161500382) TC_cm_service_during_lcs_loc_req,
* (001010000100001) TC_no_msc.

Change-Id: Ie008b5566b207b13cd562c55625bad38c09b3927
Related: OS#5337
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index e7c4599..c2ea258 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -720,6 +720,9 @@
 	pars.exp_ms_power_level := mp_exp_ms_power_level;
 	pars.mscpool.bssap_idx := bssap_idx;
 	pars.expect_tsc := BTS_TSC[0];
+	pars.imsi := f_rnd_imsi('00101'H);
+
+	log(testcasename(), ": using IMSI ", pars.imsi);
 
 	return pars;
 }