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_debug/interactive.py b/sysmocom/suites/nitb_debug/interactive.py
index bd68a57..21f4175 100755
--- a/sysmocom/suites/nitb_debug/interactive.py
+++ b/sysmocom/suites/nitb_debug/interactive.py
@@ -31,7 +31,7 @@
   elif 'wait-registered'.startswith(cmd):
     try:
       for m in modems:
-         wait(m.is_connected, nitb.mcc_mnc())
+         wait(m.is_registered, nitb.mcc_mnc())
       wait(nitb.subscriber_attached, *modems)
     except Timeout:
       print('Timeout while waiting for registration.')