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_linux_test.go

@@ -12,7 +12,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net"
 	"os"
 	"os/exec"
@@ -810,7 +810,7 @@ func TestOpenByHandleAt(t *testing.T) {
 			f := os.NewFile(uintptr(fd), "")
 			defer f.Close()
 
-			slurp, err := ioutil.ReadAll(f)
+			slurp, err := io.ReadAll(f)
 			if err != nil {
 				t.Fatal(err)
 			}