Add GitHub actions for unit testing in Petals

[peanutfun]

Changes proposed in this PR:

• Create a workflow that installs Core and Petals, and runs unit tests on Petals

This is slightly different from the Core setup in that we use a "reusable workflow" testing.yml. This way, we can trigger the workflow from the Core repository as well. This achieves compatibility testing for changes in Core.

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[chahank]

As a non-expert on github actions, I looked at it and I have nothing to add :). Thanks for making this work!

Maybe @emanuel-schmid has something to add?

[emanuel-schmid]

@chahank : not me 😄 I just merge it then we'll see.