What is likely to make the final determination is what the goal for this test server is. If you're trying to give your developers a sandbox for making new workflows and forms processes, then you can scale down on the machine specs and put SQL, Laserfiche, Forms and Workflow on the same machine.
If the goal is to double-check that your production environment will be able to sustain the load of new processes before you deploy them, then your test environment should mirror your Production environment as much as possible both in terms of machine specs and server setup.
As Travis says above, being as close to production as possible is the best way, but if cost is an issue, then consider whether performance testing is an immediate need.