ms: Rename field timer -> release_timer

Rename it so that it becomes clear what the timer is used for.

Change-Id: Id4738ea196b232858e1ec9cdc439f9b81839058f
diff --git a/tests/ms/MsTest.cpp b/tests/ms/MsTest.cpp
index 776f77c7..ea54a12 100644
--- a/tests/ms/MsTest.cpp
+++ b/tests/ms/MsTest.cpp
@@ -404,7 +404,7 @@
 	/* test MS still exists and it's idle: */
 	OSMO_ASSERT(bts_get_ms_by_tlli(bts, tlli, GSM_RESERVED_TMSI) != NULL);
 	OSMO_ASSERT(ms_is_idle(ms));
-	OSMO_ASSERT(osmo_timer_pending(&ms->timer));
+	OSMO_ASSERT(osmo_timer_pending(&ms->release_timer));
 
 	usleep(1100000);
 	osmo_timers_update();