Add Example for mocking searches in Libraries.
Add Example for mocking searches in Libraries.

I am adding an example for people who have cookbooks that
use library functions with a `Chef::Search::Query.new.search()`
call.  It's a realistic example that I am currently using in my own
environment and it also demonstrates how to use `double` to
mock a Chef API.

chefspec/chefspec,sethvargo/chefspec,chefspec/chefspec,sethvargo/chefspec