tests: Fix call-forwarding test after CFU logic update

When VoiceUnconditional is activated, all other CF is cleared and
enabled again when VoiceUnconditional is deactivated.

http://comments.gmane.org/gmane.comp.handhelds.ofono/7148 (Ofono CF
states not always correct)
0dc3515af468e23bfc0e934ed94c7ca23c06fde9 (call-forwarding: Update
conditional reporting logic)
69453afe9eeb2eba2024cd1792d25b5da61a439f (call-forwarding: Emit signals
when cfu is toggled]
tests: Fix after eliminating blocking D-Bus calls in QOfonoModem

Broken since 0d112db ([libqofono] Eliminate blocking D-Bus calls in
QOfonoModem)