msc_vlr_tests: improve cipher mode coverage

Actually call msc_vlr_set_ciph_mode() and wrap away a_iface_tx_cipher_mode()
and ranap_iu_tx_sec_mode_cmd(). Hence we'll see decisions and errors in
msc_vlr_set_ciph_mode() as well.

Change-Id: Id23bc245d4b5707edcd27c44db272fbb211bf9bd
7 files changed