Fixing broken `subscribes` notifications

Fix for https://github.com/opscode/chef/issues/2602

\cc @opscode/client-core @opscode/client-engineers 

There is more cleanup I want to do for the notifications (mainly, I don't like that `subscribes` creates a fake Resource for no reason) but I wanted to get something out there for 12.0.2 quickly.  And now there are functional tests for notifications - yay!  Although they are super ugly and fragile - I'm happy to update them to be better if anyone has feedback.

As part of testing this, I wanted to emulate the original problem on my mac so I fixed the Homebrew resource provides.
We do have Chef::Resource::Conditional already as chef/resource/conditional; that is, not everything in resource/ is a resource. This seems like it should go in there.