(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.

/cc @sodabrew