Add on-disk fixtures for SimpleSkeinOperator
Skein specs are being compared for SparkSubmitOperator and SkeinOperator, but not SimpleSkeinOperator. Lets go for consistency and add these as well. A very light refactor was done to SimpleSkeinOperator to give it a make_hook method, like SkeinOperator, so the same test can be applied to both use cases.
While reviewing the generated specs it was clear that the mjolnir upload dags were expecting the SimpleSkeinOperator to have it's _script field templated. Add that to the set of templatable fields.