use spawn for multiprocessing

brian2/tests/test_cpp_standalone.py

@@ -401,6 +401,7 @@ def test_change_parameters_multiprocessing():
 
     import multiprocessing
 
+    multiprocessing.set_start_method("spawn")
     with multiprocessing.Pool() as p:
         results = p.map(sim.run_sim, range(5))