BTS_Tests: craft a more realistic IMMEDIATE ASSIGNMENT message

In change I8b8264d28b1b1deb08774cdba58dd4c6dafe115d we modify osmo-bts
so that it will only confirm IMMEDIATE ASSIGNMENT messages. In order to
detect whether the message is an IMMEDIATE ASSIGNMENT, it has to look
into the message.

In testcase TC_pcu_data_req_imm_ass_pch we use f_rnd_octstring(23) to
generate the IMMEDIATE ASSIGNMENT message. Since osmo-bts now looks into
that message this is no longer enough. so let's generate a more realistic
IMMEDIATE ASSIGNMENT message where only the request reference and the TLLI
is randomized.

Related: OS#5927
Change-Id: Ic597154df01bdd04515edf882a567656f2a54d8c
---
M bts/BTS_Tests.ttcn
1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 5637f63..fa4e426 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -5829,7 +5829,9 @@
 
 /* Send IMM.ASS from PCU for PCH; check it appears on Um side */
 testcase TC_pcu_data_req_imm_ass_pch() runs on test_CT {
-	var octetstring imm_ass := f_rnd_octstring(23);
+	var octetstring req_ref := f_rnd_octstring(3);
+	var octetstring tlli := f_rnd_octstring(4);
+	var octetstring imm_ass := '2d063f300fc364'O & req_ref & '0000dc'O & tlli & '00232b2b2b2b'O;
 	f_init_with_pcuif();
 	f_init_l1ctl();
 	f_l1_tune(L1CTL);

