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/list_off_test.go

@@ -0,0 +1,8 @@
+//go:build !rpctest
+// +build !rpctest
+
+package itest
+
+import "github.com/lightningnetwork/lnd/lntemp"
+
+var allTestCases = []*lntemp.TestCase{}