msc: fix 2 Iu tests: use f_mm_common() instead of f_mm_auth()

Fix these tests by using f_mm_common(), which takes care of Iu auth+ciph:
TC_iu_lu_imsi_reject
TC_iu_lu_imsi_timeout_gsup

Change-Id: Id2bf160ac4e1cad4770202c6a6f1b8eeeee21d68
1 file changed