itest: assert number of outputs in offending test

The test that discovered the duplicate asset issue in the first place
would've shown the issue more clearly if we actually asserted the number
of asset UTXOs created.
We now add this assertion to make sure we only have the expected number
of assets in the asset table.

itest/addrs_test.go

@@ -649,6 +649,16 @@ func runMultiSendTest(ctxt context.Context, t *harnessTest, alice,
 		return nil
 	}, defaultTimeout/2)
 	require.NoError(t.t, err)
+
+	// We start out with two assets at Alice, one normal and one grouped. We
+	// then send it to Bob and to ourselves, with change being created. So
+	// in each round we turn one of the assets into 3 pieces (two
+	// self-transfers via addresses and one change output).
+	if runIdx == 0 {
+		AssertNumAssets(t.t, ctxt, alice, 3+1)
+	} else {
+		AssertNumAssets(t.t, ctxt, alice, 3+3)
+	}
 }
 
 // sendProof manually exports a proof from the given source node and imports it