validate:launcher: Ensure a positive job count.

The default number of jobs to use is half of the available cores rounded down, but in situations where only one core is available (such as under some VMs), this means that gst-validate-launcher defaults to using zero jobs, a case that the test-running code is not prepared to handle.

This change corrects the documentation for the --jobs option, guards against negative values both in the default setting and in argument parsing, and introduces some defensive programming to prevent other situations where the code might try to use zero jobs. It also modernizes the flooring division to use Python 3's // operator.