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Let's say you have a workflow with three steps. Step 1 is performed by any user, Step 2 is assigned to the user's manager, and Step 3 is assigned to the role "HR". Instead of hard coding these assignments, use controls in your form to set the Manager user and HR role values. Set the step assignments to the template for those controls, i.e. Step 2: {Manager} and Step 3: {Role}
Then add a business rule that checks the initial user's id - if it happens to be "designer", then set the control Manager to your test manager user (mgrTest) and set the control Role to your test role (HRtest). The else action sets these controls to the initial user's manager and the real role "HR".
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Test the workflow from your designer user account. You will receive the Task Notifications via your test user's email, and you can log in as the test user(s) to perform the tasks.
When you're ready, start the workflow from some real account. Now it should route through the real-world assignments as expected.
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