htlcswitch+funding+discovery: update mock peers to add new QuitSignal method

discovery/gossiper_test.go

@@ -2181,3 +2181,6 @@ func (p *mockPeer) PubKey() [33]byte {
 	return pubkey
 }
 func (p *mockPeer) Address() net.Addr { return nil }
+func (p *mockPeer) QuitSignal() <-chan struct{} {
+	return p.quit
+}