itest: add new RBF coop close itest

The itest has both sides try to close multiple times, each time with
increasing fee rates. We also test the reconnection case, bad RBF
updates, and instances where the local party can't actually pay for
fees.

itest/lnd_sweep_test.go

@@ -1979,6 +1979,7 @@ func testBumpForceCloseFee(ht *lntest.HarnessTest) {
 	closeTxid, err := chainhash.NewHash(pendingClose.Txid)
 	require.NoError(ht, err)
 	closingTx := ht.AssertTxInMempool(*closeTxid)
+	require.NotNil(ht, closingTx)
 
 	// The default commitment fee for anchor channels is capped at 2500
 	// sat/kw but there might be some inaccuracies because of the witness