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!