Allow test parallelisation when forkMode=always

Surefire already allows:

• forking
• parallelization within a JVM

Mixing both features would mean forking multiple JVM instead of only one.
It would allow to parallelize tests that need to be executed in a separate JVM (i.e.: with forkMode=always). Usually these tests take longer than the simple ones. In our case, 40% of the tests are executed in 4 minutes, the other 60% need two hours. So it's obviously more interesting to parallelize the former, but these ones need to fork.