avocado.job: Give an error message on an empty test list

If an empty list of tests was passed, give an error validation
message and exit. Provide a functional test for it.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #142 from lmr/empty-test-list

avocado.job: Give an error message on an empty test list