Add test scripts/sample images

[zoellner]

In order to streamline further development, we should add a few tests that prevent the layer build from succeeding while the Lambda execution later fails.

The tests should cover a few simple conversion to and narrow down any future build issues.

• sample heic file to jpg via command line. That would allow to test the libvips library separately
• sample jpeg file to webp via command line
• same two tests but within sharp

This would also help with local development using a docker container and avoid having to push WIP code to the repo in order to have it build on CodeBuild. A final test before tagging a release is still necessary since there are always some subtle differences between local testing and the CodeBuild and Lambda environments.

The test data should not end up in the final lambda layer.

Edit: After some testing I found that the examples in libheif (convert-heic) and the vips command line tool in libvips work fine and already include sample images to use.
However, the final sharp/lambda layer still doesn't work, so the focus should really be on the last bullet point above testing the libraries through sharp.