Tester over MPI

On turing, running
`time mpirun -np 120 ../nitester --engine nitg-s,nitg-g,nitg-sg,nitg-e,niti ../src/nit*.nit ../examples/*.nit *.nit`
from the `tests` directory takes 2m 46s on 120 processors to evaluate 9003 programs/engine combinations.

# Results (for the execution specified above)

* 9003 total test programs
* 5728 oks & 0ks
* 1245 fails (a lot of libraries and SDKs are missing)
* 175 no savs
* 191 fixmes
* 1494 sosos
* 125 skips
* 45 unknowns (unsupported results: skip soso, changed, etc.)

# Features and todo

- [x] Uses OpenMPI
- [x] Supports all engines through tests.sh
- [x] Collects and classify results
- [x] Works on the Turing cluster
- [x] Optimized to avoid overloading NFS
- [ ] Correctly detect the all the results (no more unknowns)
- [ ] Use with Jenkins (see what we can do with the generated XML files)
- [ ] Gather and keep failed results temporary files
- [ ] Be more customizable to run on different clusters
- [ ] Integrate some features from tests.sh
J'ai eu un bug sur mon cluster local où un seul probe ne me donnait pas les infos du prochain message. C'est assez problématique, mais un double probe suffit à éviter le problème.