Fix the user-triggered termination test

[therault]

• Make sure the parsec context is started only after the taskpool is created as the taskpool will register active message handlers for the first time via the user-triggered option in PTG
• Correctly compute the number of local tasks instead of assuming a run at a given world size; compute that number on all ranks and check that all ranks have a correct value
• have all MPI rank return the same return code for consistency
• Free the taskpool when it's not used anymore to remove assertion.