ms_srs: refactor UE state getter functions

rename UE helper functions to query RRC connected/idle state
and adapt tests accordingly

also add helper to retrieve assinged IP address

Change-Id: I6cd057e34b4df6a1a73695355dd6406d7e039546
diff --git a/sysmocom/suites/4g/ping.py b/sysmocom/suites/4g/ping.py
index c81a35d..0fb2edb 100755
--- a/sysmocom/suites/4g/ping.py
+++ b/sysmocom/suites/4g/ping.py
@@ -16,7 +16,7 @@
 
 ue.connect(enb)
 print('waiting for UE to attach...')
-wait(ue.is_connected, None)
+wait(ue.is_rrc_connected)
 print('UE is attached')
 
 proc = ue.run_netns_wait('ping', ('ping', '-c', '10', epc.tun_addr()))