all: remove ioutil usage from tests

This removes the remaining (and trivial) use of deprecated ioutil
package from test files.

Replacements are easy:

	ioutil.ReadAll -> io.ReadAll
	ioutil.ReadDir -> os.ReadDir
	ioutil.ReadFile -> os.ReadFile
	ioutil.WriteFile -> os.WriteFile

While at it, simplify some error reporting.

Change-Id: I60a242fd3c08d8fe571a18f16716439a9acdd59d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526299
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>

windows/svc/svc_test.go

@@ -9,7 +9,6 @@ package svc_test
 
 import (
 	"fmt"
-	"io/ioutil"
 	"math/rand"
 	"os"
 	"os/exec"
@@ -202,7 +201,7 @@ func TestIsWindowsServiceWhenParentExits(t *testing.T) {
 		if isSvc {
 			msg = "IsWindowsService returns true when not running in a service."
 		}
-		err = ioutil.WriteFile(dumpPath, []byte(msg), 0644)
+		err = os.WriteFile(dumpPath, []byte(msg), 0644)
 		if err != nil {
 			// We cannot report this error. But main test will notice
 			// that we did not create dump file.
@@ -232,7 +231,7 @@ func TestIsWindowsServiceWhenParentExits(t *testing.T) {
 				t.Fatal("timed out waiting for child output file to be created.")
 			}
 		}
-		childOutput, err := ioutil.ReadFile(childDumpPath)
+		childOutput, err := os.ReadFile(childDumpPath)
 		if err != nil {
 			t.Fatalf("reading child output failed: %v", err)
 		}