itest+lntest: fix itest re the new block driven behavior

itest/lnd_wipe_fwdpkgs_test.go

@@ -114,6 +114,12 @@ func testWipeForwardingPackages(ht *lntest.HarnessTest) {
 	pendingAB = pending.Channel
 	require.Zero(ht, pendingAB.NumForwardingPackages)
 
+	// Alice should one pending sweep.
+	ht.AssertNumPendingSweeps(alice, 1)
+
+	// Mine a block to trigger the sweep.
+	ht.MineBlocks(1)
+
 	// Mine 1 block to get Alice's sweeping tx confirmed.
 	ht.MineBlocksAndAssertNumTxes(1, 1)