itest: graph sql migration itest

This commit adds a basic itest that is run under the `test_native_sql`
flag. It tests that the migration is idempotent and that the node cant
be restarted without the `db.use-native-sql` flag once the graph
migration has been run.

itest/lnd_invoice_migration_test.go

@@ -23,6 +23,7 @@ import (
 
 func openKVBackend(ht *lntest.HarnessTest, hn *node.HarnessNode) kvdb.Backend {
 	sqlbase.Init(0)
+
 	var (
 		backend kvdb.Backend
 		err     error