Write a test case

[hoijui]

Something like:

1. generating/downloading a random image
2. embedding a random message into it
3. use histogram to analyze the image and check if containment of a hidden message is detectable
4. extracting the message again
5. check if the extracted message equals the original one

put all of this into .github/workflows/full-check.yml

[eribertomota]

This task is 99% done. The problem is histogram always returns:

histogram: malloc: Cannot allocate memory