How to add a "tests passing" badge to GitHub and why you should do it

Testing

This is an example of a badge:
https://github.com/eugenefvdm/project/actions/workflows/run-tests.yml/badge.svg

What's important to note apart from the `project` is `run-tests.yml`. If you're working on many projects, and to retain consistency, it is therefore imperative to name your test runners the same.

Here is the full markdown:

![Tests](https://github.com/eugenefvdm/project/actions/workflows/run-tests.yml/badge.svg)
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