Make on_main_thread test more reliable

Author: Liozou

test/Core.jl

@@ -838,8 +838,9 @@ end
 end
 
 @testitem "on_main_thread" begin
-    task = Threads.@spawn PythonCall.C.on_main_thread() do; Threads.threadid(); end
-    @test fetch(task) == 1
+    refid = PythonCall.C.on_main_thread() do; Threads.threadid(); end
+    tasks = [Threads.@spawn(PythonCall.C.on_main_thread() do; Threads.threadid(); end) for _ in 1:20]
+    @test all(t -> fetch(t) == refid, tasks)
     @test_throws DivideError redirect_stderr(devnull) do
         PythonCall.C.on_main_thread() do
             throw(DivideError())