Add 'jobs' argument for 'dzil test'

This passes a jobs argument to TestRunners so they can set harness
options (or whatever) accordingly.
add job_count attribute for test runners
Move file writing out of argument loop

Previously, on a second entry the file generated in the first would be
written again, which resulted in an error. Now it is written after all
files have been generated.
credit for cc759e1
fix the placement of variables in loops
v5.014

        - Added 'jobs' argument for 'dzil test' for parallel testing (thanks,
          David Golden!)
        - add default_jobs attribute to TestRunner role
        - fix the behavior of 'dzil add' with more than one file
          (thanks, Leon Timmermans!)