compute execution time

OK program1: compute execution time with 1 thread without block/unblock
OK program2: compute execution time with 2 threads without block/unblock
OK program3: compute execution time with 3 threads without block/unblock
OK program4: compute execution time with 1 thread, 1 call to block (nanosleep), 1 call to unblock
OK program5: compute execution time with 1 thread, 2 call to block, 2 call to unblock
PAS OK program6: compute execution time with 2 thread, 1 thread 1 call block/unblock