Add test job in ci GitHub Actions workflow

[victorpopkov]

Problem

Similar to dstmodders/docker-imagemagick#19.

Once #13 gets solved, it would be nice to run these as part of our CI workflow and upload generated tests/result/ directory as artifacts.

Proposed solution

Just extend .github/workflows/ci.yml to introduce test job, which runs the tests/tests.sh script and upload the result as artifacts.

Progress

• Add the test job in .github/workflows/ci.yml that runs the tests/tests.sh script.
• Add support for uploading tests/result/ directory as artifacts.