Adding read accessor for @tmpdir in ChefSpec::Berkshelf

This commit adds a method to access the tmpdir variable in ChefSpec::Berkshelf.

ChefSpec::Berkshelf stores all it's cookbooks in a tmp directory.
This makes the use of ```ChefSpec::Coverage.filters``` impossible when Berkshelf is used with ChefSpec.

Adding this method allows for the filtering of Berkshelf cookbooks in the coverage report.