JobWrapper: add more unit test

[aldbr]

There is currently 0 unit test around the jobwrapper. As it's a critical piece, we need to add much more tests.

Plan:

• List all the tests that could be interesting to have: general and corner cases (we want to test the public methods - note: pre/post process should be public)
• Implement them

Note: we prefer having clear than very well optimized tests. For instance, it's generally clearer to have multiple tests with hardcoded values describing a use case, than having a parametrized tests with different variables and conditions.