tests: add test case "disconnecting target peer"

Issue: 139

This commit contains test case "disconnecting target peer" (second test case) which takes two
connected peers, then checks via assert method one connection exists,
then disconnects this remote peer by passing pubKey parameter (just some string) into RPC-call method. Then checks 0
connection exists, and then connects disconnected peer for passing
further tests, and then checks one connection exists.

networktest.go

@@ -664,7 +664,6 @@ func (n *networkHarness) SetUp() error {
 	// Swap out grpc's default logger with out fake logger which drops the
 	// statements on the floor.
 	grpclog.SetLogger(&fakeLogger{})
-
 	// Start the initial seeder nodes within the test network, then connect
 	// their respective RPC clients.
 	var wg sync.WaitGroup
@@ -847,6 +846,25 @@ func (n *networkHarness) ConnectNodes(ctx context.Context, a, b *lightningNode)
 	}
 }
 
+// DisconnectNodes disconnects node a from node b by sending RPC message
+// from a node to b node
+func (n *networkHarness) DisconnectNodes(ctx context.Context, a, b *lightningNode) error {
+	bobInfo, err := b.GetInfo(ctx, &lnrpc.GetInfoRequest{})
+	if err != nil {
+		return err
+	}
+
+	req := &lnrpc.DisconnectPeerRequest{
+		PubKey: bobInfo.IdentityPubkey,
+	}
+
+	if _, err := a.DisconnectPeer(ctx, req); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // RestartNode  attempts to restart a lightning node by shutting it down
 // cleanly, then restarting the process. This function is fully blocking. Upon
 // restart, the RPC connection to the node will be re-attempted, continuing iff