Implement 3 different runner for tests (threads, sequential and processes).

Ignore-this: eedc6a1aacb663a5ad6cb6f08ed3ab9e

Split the runner from OUnitCore. We have now the choice of runner:
- seq(uential): the default runner,
- threads: a runner that uses threads, don't improve speed expect when a lot of
I/O or external programs involved.
- processes: a process based runner that should increase parallelism of
testing.

Thanks to Thomas Wickham for its patch, which has been used and reworked for
this commit.