contractcourt+itest: fix lingering pending channel after breach

This fixes an issue where the contract court could leave a completely
swept commit tx unresolved if it was swept by the remote party.

This could happen if (our) commit tx just published was actually a
previously revoked state, in which case the remote party would claim the
funds via a justice transaction.

This manifested itself in the testRevokedCloseRetribution integration
test where at the end of the test Bob was left with a pending channel
that never resolved itself.

lntest/itest/lnd_test.go

@@ -7586,6 +7586,15 @@ func testRevokedCloseRetribution(net *lntest.NetworkHarness, t *harnessTest) {
 	}
 
 	assertNodeNumChannels(t, carol, 0)
+
+	// Mine enough blocks for Bob's channel arbitrator to wrap up the
+	// references to the breached channel. The chanarb waits for commitment
+	// tx's confHeight+CSV-1 blocks and since we've already mined one that
+	// included the justice tx we only need to mine extra DefaultCSV-2
+	// blocks to unlock it.
+	mineBlocks(t, net, lntest.DefaultCSV-2, 0)
+
+	assertNumPendingChannels(t, net.Bob, 0, 0)
 }
 
 // testRevokedCloseRetributionZeroValueRemoteOutput tests that Dave is able