itest: remove direct reference to stanby nodes

Prepares the upcoming refactor. We now never call `ht.Alice` directly,
instead, we always init `alice := ht.Alice` so it's easier to see how
they are removed in a following commit.

itest/lnd_estimate_route_fee_test.go

@@ -109,7 +109,7 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 		},
 	)
 
-	bobsPrivChannels := ht.Bob.RPC.ListChannels(&lnrpc.ListChannelsRequest{
+	bobsPrivChannels := mts.bob.RPC.ListChannels(&lnrpc.ListChannelsRequest{
 		PrivateOnly: true,
 	})
 	require.Len(ht, bobsPrivChannels.Channels, 1)