Probably the easiest would be a content filter.
Under Mail Policies/Incoming Content Filters or Outgoing Content Filters as appropriate, create a filter with a condition of the Recipient = your test address, action of Send to Alternate Destination Host.
Then in your mail policy, enable this content filter.
If this is an actual user real address, then you'll want to break things up to take advantage of splintering.
Create the content filter with NO condition.
Then in the Mail Policies/Incoming mail policies or Outgoing Mail policies, create a new policy, Click on "Add User" and set the "Following recipients" to the appropriate email addresses.
Enable whatever security filters you need in that policy or use the defaults, except for Content filters where you add this new filter.
This is so if someone sends to your test user and others in the same email, the mail to your test user goes to the new route, other users will go via the old routes.
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