Test nitvm

Add the engine `nitvm` in tests.sh so the developers of the vm can rely on the complex test framework.

1. compile nitvm (put it in src/ or bin/)
2. `$ cd tests`
3. `$ ./tests.sh --engine nitvm base*.nit` for some tests
4. `$ ./testfull.sh --engine nitvm` for all tests (long!)

Note: currently some tests (typing related) fails.
Note2: once the vm pass all tests, a new PR is needed so it is automatically built and tested.