Allow a per-process time limit to be specified
Allow a per-process time limit to be specified

If the key `time_limit` is specified in `.zunit.yml`, then the test is
executed in a child process. The child process is killed if it is still
running after `$time_limit` seconds have passed, and the test is marked
as an error in the results.

Fix #39

zunit-zsh/zunit,molovo/zunit