L1CTL: Add support for CCCH_MODE_COMBINED_CBCH

In OsmocomBB/trxcon Change-Id Ia94ebf22a2ec439dfe1f31d703b832ae57b48ef2
we introduced a new mode CCCH_MODE_COMBINED_CBCH to indicate that the
channel combination is a CCCH+SDCCH/4 with one SDCCH stolen for CBCH.

Let's make sure we actually use that mode in our CBCH related tests

Change-Id: I27ee2c81bec7175c1ea09d4f3f6037f2866fe242
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 1c20657..47c16c2 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -451,8 +451,8 @@
 	}
 }
 
-friend function f_l1_tune(L1CTL_PT L1CTL) {
-	f_L1CTL_FBSB(L1CTL, { false, mp_trx0_arfcn }, CCCH_MODE_COMBINED, mp_rxlev_exp);
+friend function f_l1_tune(L1CTL_PT L1CTL, L1ctlCcchMode ccch_mode := CCCH_MODE_COMBINED) {
+	f_L1CTL_FBSB(L1CTL, { false, mp_trx0_arfcn }, ccch_mode, mp_rxlev_exp);
 }
 
 private function f_trxc_connect() runs on ConnHdlr {