Create test project and some basic tests

[Issung]

Nowadays writing code without tests makes me nervous.
Tests are a great way to:

• Define the contract of what your code should do.
• Ensure you don't break anything.
• Drive your development (in certain cases).
• Make intended behaviour clearer to peers.

My preferred testing framework is xunit.

• You will need to make a new project using this short guide: https://xunit.net/docs/getting-started/netfx/visual-studio.
• Call the project GChan.Test.
• Add the original GChan project as a dependency.
• Add some basic tests for the ImageLink class, test that generating with each different format (and an unexpected format) gives the expected output.
• Look for other classes than can be easily tested.

CICD:

• If doing this after making the GitHub Actions pipeline, add testing of this project to the pipeline.
• If doing this before that issue, update the other issue to say "add unit testing to pipeline".