cgen: fix gen wrong for thread wrappers, when spawn with anon-fn array args(fix #19425)

[shove70]

1. Fixed C gen error when spawning function with mutable type #19425
2. Add tests.

module main

import time
import io
import os

interface Message {
	a int
}

struct Impl {
	a int
}

type HookFunc = fn (Message) ?Message

struct TestStruct {
	fns []HookFunc
}

fn (ts TestStruct) run(mut writer io.Writer) {
	spawn handle_fns(ts.fns, mut writer)
}

fn handle_fns(fns []HookFunc, mut writer io.Writer) {
	mut message := Message(Impl{a: 1})
	for f in fns {
		message = f(message) or { continue }
	}
	println(message)
}

fn main() {
	stack := TestStruct{
		fns: [
			fn (m Message) ?Message {
				return Message(Impl{a: m.a + 1})
			},
			fn (m Message) ?Message {
				return none
			}
		]
	}
	stack.run(mut os.stdout())
	time.sleep(2 * time.second)
}

outputs:

Message(Impl{
    a: 2
})