multi: move itest out of lntest

This commit moves all the test cases living in `itest` out of `lntest`,
further making `lntest` an independent package for general testing.

itest/lnd_no_etcd_dummy_failover_test.go

@@ -0,0 +1,10 @@
+//go:build !kvdb_etcd
+// +build !kvdb_etcd
+
+package itest
+
+import "github.com/lightningnetwork/lnd/lntemp"
+
+// testEtcdFailover is an empty itest when LND is not compiled with etcd
+// support.
+func testEtcdFailover(ht *lntemp.HarnessTest) {}