itest+lntest: migrate lntemp to lntest

This commit performs the takeover that `lntemp` is now promoted to be
`lntest`, and the scaffolding is now removed as all the refactoring is
finished!

itest/lnd_no_etcd_dummy_failover_test.go

@@ -3,8 +3,8 @@
 
 package itest
 
-import "github.com/lightningnetwork/lnd/lntemp"
+import "github.com/lightningnetwork/lnd/lntest"
 
 // testEtcdFailover is an empty itest when LND is not compiled with etcd
 // support.
-func testEtcdFailover(ht *lntemp.HarnessTest) {}
+func testEtcdFailover(ht *lntest.HarnessTest) {}