[COOK-3011] Apt Source Test Causes Chef Run to Fail on Ubuntu

Seeing an error when running this cookbook on Ubuntu Precise:

 2) Failure:
rabbitmq::default#test_0001_uses the rabbitmq apt source on debian family [/var/chef/minitest/rabbitmq/default_test.rb:27]:
Expected path '/etc/apt/sources.list.d/rabbitmq-source.list' to exist

The test seems to be looking for a non-existent apt source repo. I've updated the test to look for the downloaded .deb file instead