Restore from scratch each time.

Write a custom sync program or a rather complex SSIS package to do the same thing. This will have a non-trivial impact on production

Replication's the usual suggestion for this and it's the only easy way of doing it. Probably your best bet is to restore the full and then the latest diff. Doe the test server really need to be up to the hour accurate? If it can be as production was the previous night, then backup/restore is feasible.