Require unit tests to be written for custom scripts used by pipelines

[adlersantos]

Description

We expect more and more pipelines to include custom scripts (e.g. transforming CSV files, reading .shp or .nc files, scraping websites, and so on) and we need to start requiring contributors to write unit tests for them.

This will help minimize issues during internal deployment (to the Public Datasets Program) that we (Googlers) could've caught further upstream.

Some items to consider are

• Where should contributors place the test files,
• The unit testing conventions or guides they need to follow
• Updating the README for unit testing requirements
• Adding all the unit tests as part of GitHub Actions

Checklist

• [x] I created this issue in accordance with the Code of Conduct.
• [x] This issue is appropriately labeled.