BSC_Tests: use isvalue() instead of isbound()

Currently we use isbound() in f_start_handler() to check if the BTS
which we want to connect is indeed populated. However. isbound()
seems never become true in this particular situation.

- Use isvalue() instead of isbound()

Change-Id: I25ddd55b7626087570311999b85ec7632b162c06
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index 8d397f7..dc266ab 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -1348,7 +1348,7 @@
 	connect(vc_conn:MGCP_PROC, vc_MGCP:MGCP_PROC);
 	connect(vc_conn:RSL, bts[0].rsl.vc_RSL:CLIENT_PT);
 	connect(vc_conn:RSL_PROC, bts[0].rsl.vc_RSL:RSL_PROC);
-	if (isbound(bts[1])) {
+	if (isvalue(bts[1])) {
 		connect(vc_conn:RSL1, bts[1].rsl.vc_RSL:CLIENT_PT);
 		connect(vc_conn:RSL1_PROC, bts[1].rsl.vc_RSL:RSL_PROC);
 	}