tests/test_zpm: add tests for util functions

This adds tests for `_generate_job_desc` and `find_project_root` in the
`zpmlib.zpm` module.
zpm: add docstrings to some util funcs

This add docstrings to `_generate_job_desc` and `find_project_root`.
Merge pull request #114 from larsbutler/improve-test-coverage

Improve test coverage