tests: Replace is_connected() with is_registered()

API was renamed and old name deprecated, let's use it everywhere.

Change-Id: Ic91dddcfd1a5a17c35d15a8a14628b3923229523
diff --git a/sysmocom/suites/nitb_netreg/register.py b/sysmocom/suites/nitb_netreg/register.py
index fd171b3..4977f66 100755
--- a/sysmocom/suites/nitb_netreg/register.py
+++ b/sysmocom/suites/nitb_netreg/register.py
@@ -18,5 +18,5 @@
 
 print(ms.info())
 
-wait(ms.is_connected, nitb.mcc_mnc())
+wait(ms.is_registered, nitb.mcc_mnc())
 wait(nitb.subscriber_attached, ms)