io: use runtime.AddCleanup instead of runtime.SetFinalizer

Replace the usage of runtime.SetFinalizer with runtime.AddCleanup.

Updates #70907

Change-Id: Id604ca44ea67dcf8f87797e27347c6f4e9ad0b86
Reviewed-on: https://go-review.googlesource.com/c/go/+/638556
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Carlos Amedee <[email protected]>
TryBot-Bypass: Carlos Amedee <[email protected]>

Author: cagedmantis

src/io/multi_test.go

@@ -332,9 +332,7 @@ func TestMultiReaderFreesExhaustedReaders(t *testing.T) {
 		buf1 := bytes.NewReader([]byte("foo"))
 		buf2 := bytes.NewReader([]byte("bar"))
 		mr = MultiReader(buf1, buf2)
-		runtime.SetFinalizer(buf1, func(*bytes.Reader) {
-			close(closed)
-		})
+		runtime.AddCleanup(buf1, func(ch chan struct{}) { close(ch) }, closed)
 	}()
 
 	buf := make([]byte, 4)