Generate SI2bis Rest Octets

According to the spec it's an empty 1-byte element reserved for future
extension but we still have to generate padding properly. Add stub
function similar to the used for SI2ter and adjust test output
accordingly.

Related: OS#2711
Change-Id: I3c278c57880a173df3c4648c9724339d23ce94fd
diff --git a/tests/gsm0408/gsm0408_test.ok b/tests/gsm0408/gsm0408_test.ok
index 868cd9b..536287c 100644
--- a/tests/gsm0408/gsm0408_test.ok
+++ b/tests/gsm0408/gsm0408_test.ok
@@ -203,7 +203,7 @@
 generated valid SI2quater [05/05]: [23] 59 06 07 4a a0 04 86 59 84 26 53 97 65 60 2b 2b 2b 2b 2b 2b 2b 2b 2b 
 Testing if BA-IND is set as expected in SI2xxx and SI5xxx
 SI2: 59 06 1a 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
-SI2bis: 59 06 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
+SI2bis: 59 06 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 2b 
 SI2ter: 59 06 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 2b 2b 2b 2b 
 SI5: 06 1d 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
 SI5bis: 06 05 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00