itest: fix flake in testAnchorThirdPartySpend

When mining lots of blocks in the itest, the subsystems can be out of
sync in terms of the best block height. We now assert the num of pending
sweeps on Alice's node to give her more time to sync the blocks,
essentially behaving like a `time.Sleep` as in reality, the blocks would
never be generated this fast.

Author: yyforyongyu

itest/lnd_onchain_test.go

@@ -564,7 +564,13 @@ func testAnchorThirdPartySpend(ht *lntest.HarnessTest) {
 	//
 	// TODO(yy): also check the restart behavior of Alice.
 	const anchorCsv = 16
-	ht.MineEmptyBlocks(anchorCsv - defaultCSV)
+	blocks := anchorCsv - defaultCSV
+
+	// Mine empty blocks and check Alice still has the two pending sweeps.
+	for i := 0; i < blocks; i++ {
+		ht.MineEmptyBlocks(1)
+		ht.AssertNumPendingSweeps(alice, 2)
+	}
 
 	// Now that the channel has been closed, and Alice has an unconfirmed
 	// transaction spending the output produced by her anchor sweep, we'll