(maint) refactor Report.create_from_yaml_file test

Prior to this commit, the tests for Report.create_from_yaml_file were
commented out due to weirdness around Mocha and stubbing File.read. This
commit does a minor refactor of Report.create_from_yaml_file and
associated tests to allow expectations and stubbing to be done on our
local methods rather than on File.read and File.unlink. We can safely
assume that File.read and File.unlink work the way they ought.