Rails - Sep 2009 - using _url helper in unit test

I have a actionmailer model that send the user a URL to the website
for account activation.  In my unit tests for the model I want to
ensure that the correct URL is being generated.  In the test I have
the following:

assert sent.body.include? new_account_activation_url(:activation_code
=> user.perishable_token)

When I run the test, I get  undefined method
''new_account_activation_url''. 
''new_account_activation_url'' works fine
in the model so I know it is not a routing problem.  I assume that I
need to include some helper to get this to work but I am not sure what
to include.

Any ideas?

Thanks!