DAG testing documentation

[AetherUnbound]

Description

It would be ideal if we had basic "DAG PR testing" documentation. This would prevent us from having to redefine the testing steps for each PR, and allow us to point to a link of the specific common steps required (with addendums & caveats as needed). Some examples might be:

• Basic testing instructions (just recreate && just test, etc.)
• Run a provider DAG which requires marking as success (a la Add new columns to MediaStore and database openverse-catalog#196)
• Run a provider DAG with ingestion_limit set and ensure it runs to completion without intervention
• Set up a local API stack in order to run the data refresh DAG (a la Add data refresh to Airflow openverse-catalog#397)

Implementation

• 🙋 I would be interested in implementing this feature.