Add tests

[victorpopkov]

Problem

Similar to dstmodders/docker-imagemagick#14:

Currently, we don't have tests to ensure everything works as expected.

Proposed solution

Add a simple script that uses tests similar to the dstmodders/ktools ones:
https://github.com/dstmodders/ktools/blob/main/test/test.sh

Progress

• Add tests/tests.sh.