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>

unix/syscall_zos_test.go

@@ -10,7 +10,7 @@ package unix_test
 import (
 	"flag"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net"
 	"os"
 	"os/exec"
@@ -258,7 +258,7 @@ func TestPassFD(t *testing.T) {
 	f := os.NewFile(uintptr(gotFds[0]), "fd-from-child")
 	defer f.Close()
 
-	got, err := ioutil.ReadAll(f)
+	got, err := io.ReadAll(f)
 	want := "Hello from child process!\n"
 	if string(got) != want {
 		t.Errorf("child process ReadAll: %q, %v; want %q", got, err, want)
@@ -628,7 +628,7 @@ func TestChroot(t *testing.T) {
 		t.Fatalf("Chroot: %s", err.Error())
 	}
 	// check if tempDir contains test file
-	files, err := ioutil.ReadDir("/")
+	files, err := os.ReadDir("/")
 	if err != nil {
 		t.Fatalf("ReadDir: %s", err.Error())
 	}