Fix MS ending up with assigned imsi 000

The whole paging path and data structre is cleaned up.
New MS helpers ms_imsi_is_valid() and ms_paging_group() are introduced
to help in the process and keep implementation details inside GprsMs
class.

Related: OS#5303
Change-Id: I4c0838b26ede58e4b711410eee2a8e4f71e9414b
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 8700347..9bea528 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -1718,7 +1718,7 @@
 	OSMO_ASSERT(ms != NULL);
 	OSMO_ASSERT(ms_dl_tbf(ms) != NULL);
 
-	if (imsi[0] && strcmp(imsi, "000") != 0) {
+	if (imsi[0] != '\0') {
 		ms2 = bts_ms_by_tlli(bts, tlli, GSM_RESERVED_TMSI);
 		OSMO_ASSERT(ms == ms2);
 	}