cgen: fix assign the spawn or go expr to the []thread(fix #19365)

[shove70]

1. Fix cgen error: assign the spawn or go expr to the []thread #19365
2. Add tests.

fn main() {
	mut threads := []thread{len: 1}

	threads[0] = spawn fn () {
		a := 1
		dump(a)
		assert true
	}()

	threads.wait()
}

output:

[a.v:6] a: 1