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/syscall_windows_test.go

@@ -10,7 +10,6 @@ import (
 	"debug/pe"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"math/rand"
 	"os"
 	"path/filepath"
@@ -667,14 +666,14 @@ func TestWinVerifyTrust(t *testing.T) {
 	// Now that we've verified the legitimate file verifies, let's corrupt it and see if it correctly fails.
 
 	corruptedEvsignedfile := filepath.Join(t.TempDir(), "corrupted-file")
-	evsignedfileBytes, err := ioutil.ReadFile(evsignedfile)
+	evsignedfileBytes, err := os.ReadFile(evsignedfile)
 	if err != nil {
 		t.Fatalf("unable to read %s bytes: %v", evsignedfile, err)
 	}
 	if len(evsignedfileBytes) > 0 {
 		evsignedfileBytes[len(evsignedfileBytes)/2-1]++
 	}
-	err = ioutil.WriteFile(corruptedEvsignedfile, evsignedfileBytes, 0755)
+	err = os.WriteFile(corruptedEvsignedfile, evsignedfileBytes, 0755)
 	if err != nil {
 		t.Fatalf("unable to write corrupted ntoskrnl.exe bytes: %v", err)
 	}