Add on-disk based fixture files containing skein specs for all tasks

Generalizes and moves the for_each_task and render_task implementations from the search teams airflow tests up to the top level conftest. Uses this functionality to add fixtures that demonstrate the current skein specs that are submitted in production. While this tries to be accurate, the recorded specs are not 100% the same as production (ex: keytab paths). Future work could try and clean up these minor inconsistencies.

MR contains multiple patches intended to be merged without squashing. Each patch passes the test suite and builds upon prior patches to provide the desired functionality.