test: fix flaky test for issue24491

runtime.GC() doesn't guarantee the finalizer has run, so use a channel
instead to make sure finalizer was run in call to "after()".

Fixes #41361

Change-Id: I69c801e29aea49757ea72c52e8db13239de19ddc
Reviewed-on: https://go-review.googlesource.com/c/go/+/254401
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>

Author: cuonglm

test/fixedbugs/issue24491b.go

@@ -11,33 +11,33 @@ package main
 
 import (
 	"runtime"
-	"sync/atomic"
 	"unsafe"
 )
 
-var done uint32
+var done = make(chan bool)
 
 func setup() unsafe.Pointer {
 	s := "ok"
-	runtime.SetFinalizer(&s, func(p *string) { atomic.StoreUint32(&done, 1) })
+	runtime.SetFinalizer(&s, func(p *string) { close(done) })
 	return unsafe.Pointer(&s)
 }
 
 //go:noinline
 //go:uintptrescapes
 func before(p uintptr) int {
 	runtime.GC()
-	if atomic.LoadUint32(&done) != 0 {
+	select {
+	case <-done:
 		panic("GC early")
+	default:
 	}
 	return 0
 }
 
 func after() int {
 	runtime.GC()
-	if atomic.LoadUint32(&done) == 0 {
-		panic("GC late")
-	}
+	runtime.GC()
+	<-done
 	return 0
 }