Enhance tests to fail programmatically

[eine]

Currently, tests consist on printing environment variables. Therefore, human inspection is required to find regression issues. For example, the default shell changed, and we didn't spot it (see #16). Tests should be enhanced to prevent similar issues in the future. On the one hand, #15 schedules jobs to be executed periodically. On the other hand, test.sh should not only show but also check the content of environment variables.