For some reason, starting processes with spawn causes the following error:
```
Traceback (most recent call last):
File "/home/pierre/dev/bitcoin-abc/test/functional/test_framework/test_framework.py", line 136, in main
self.run_test()
File "/home/pierre/dev/bitcoin-abc/test/functional/interface_rpc.py", line 124, in run_test
self.test_work_queue_exceeded()
File "/home/pierre/dev/bitcoin-abc/test/functional/interface_rpc.py", line 107, in test_work_queue_exceeded
p.start()
File "/usr/lib/python3.10/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
File "/usr/lib/python3.10/multiprocessing/context.py", line 224, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "/usr/lib/python3.10/multiprocessing/context.py", line 288, in _Popen
return Popen(process_obj)
File "/usr/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 32, in __init__
super().__init__(process_obj)
File "/usr/lib/python3.10/multiprocessing/popen_fork.py", line 19, in __init__
self._launch(process_obj)
File "/usr/lib/python3.10/multiprocessing/popen_spawn_posix.py", line 47, in _launch
reduction.dump(process_obj, fp)
File "/usr/lib/python3.10/multiprocessing/reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
File "/home/pierre/dev/bitcoin-abc/test/functional/test_framework/test_node.py", line 865, in __call__
return self.cli.send_cli(self.command, *args, **kwargs)
File "/home/pierre/dev/bitcoin-abc/test/functional/test_framework/test_node.py", line 938, in send_cli
raise JSONRPCException(dict(code=int(code), message=message))
test_framework.authproxy.JSONRPCException: Method not found (-32601)
```
Spawn is the default on Mac OS, so we need to explicitely set it to "fork"