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/list_off_test.go

@@ -2,6 +2,6 @@
 
 package itest
 
-import "github.com/lightningnetwork/lnd/lntemp"
+import "github.com/lightningnetwork/lnd/lntest"
 
-var allTestCases = []*lntemp.TestCase{}
+var allTestCases = []*lntest.TestCase{}