add bsc.vty: always keep a BSC VTY connection ready

The BSC's VTY port will be used to trigger manual handover, and to
retrieve a list of active lchans from the BSC, in the upcoming
handover_2G test suite, I0b2671304165a1aaae2b386af46fbd8b098e3bd8.

Change-Id: I06652db04fc9e48748f3c2196334f5352e9cc48a
diff --git a/src/osmo_gsm_tester/obj/osmo_vty.py b/src/osmo_gsm_tester/obj/osmo_vty.py
index 3f8abf7..6fee5dc 100644
--- a/src/osmo_gsm_tester/obj/osmo_vty.py
+++ b/src/osmo_gsm_tester/obj/osmo_vty.py
@@ -38,6 +38,9 @@
 
        Using 'with' ensures that the connection is closed again.
        There should not be nested 'with' statements on this object.
+
+       Note that test env objects (like tenv.bsc()) may keep a VTY connected until the test exits. A 'with' should not
+       be used on those.
     '''
 
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