providers/heroku: fix drain test

cc/ @bmarini

Basically, the attr check we have in our tests is looking at resources
that we specify. It's basically check that "if I send this, I get this"
from the external resource.

In this case, we don't know what the token will be, so we can't
do it that way. I replaced it with a "make sure this is more than an
empty string" assertion.