ocp-build: more powerful tests

This version of ocp-build was used to parallize the testsuite
of the OCaml compiler (not yet complete).

The "Tests" chapter of ocp-build user-manual is complete.
ocp-build: more small improvements before release

- more test options (-benchmarks arg, test_benchmarked)
- display args and errors: cut lines at 80
- don't error out on package conflicts