ts_TRXC_FAKE_RSSI(): define RSSI range, allow signed values

Change-Id: I8944e690c0da648eae3005777f2487c77b66a17b
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 5481838..37a5e3c 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -467,7 +467,7 @@
 	g_bts_trxc_conn_id := res.connId;
 }
 
-private function f_trxc_fake_rssi(uint8_t rssi) runs on ConnHdlr {
+private function f_trxc_fake_rssi(TRXC_RSSI rssi) runs on ConnHdlr {
 	var TrxcMessage ret;
 	ret := f_TRXC_transceive(BTS_TRXC, g_bts_trxc_conn_id, valueof(ts_TRXC_FAKE_RSSI(rssi)));
 }