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_smpp/esme_ms_sms_transaction.py b/sysmocom/suites/nitb_smpp/esme_ms_sms_transaction.py
index 2963b6e..ade8054 100755
--- a/sysmocom/suites/nitb_smpp/esme_ms_sms_transaction.py
+++ b/sysmocom/suites/nitb_smpp/esme_ms_sms_transaction.py
@@ -28,7 +28,7 @@
 
 ms.log_info()
 print('waiting for modem to attach...')
-wait(ms.is_connected, nitb.mcc_mnc())
+wait(ms.is_registered, nitb.mcc_mnc())
 wait(nitb.subscriber_attached, ms)
 
 print('sending first sms...')