all: simplify code by using modern Go constructs

Generated using modernize by running:

    go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Change-Id: Ifc7d61cf6735cc53f2bdf890a338961f55075af5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/661975
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

unix/xattr_test.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"strings"
 	"testing"
 
@@ -62,13 +63,7 @@ func TestXattr(t *testing.T) {
 		// name and Listxattr doesn't return the namespace prefix.
 		xattrWant = strings.TrimPrefix(xattrWant, "user.")
 	}
-	found := false
-	for _, name := range xattrs {
-		if name == xattrWant {
-			found = true
-			break
-		}
-	}
+	found := slices.Contains(xattrs, xattrWant)
 
 	if !found {
 		t.Errorf("Listxattr did not return previously set attribute %q in attributes %v", xattrName, xattrs)
@@ -170,13 +165,7 @@ func TestFdXattr(t *testing.T) {
 		// name and Listxattr doesn't return the namespace prefix.
 		xattrWant = strings.TrimPrefix(xattrWant, "user.")
 	}
-	found := false
-	for _, name := range xattrs {
-		if name == xattrWant {
-			found = true
-			break
-		}
-	}
+	found := slices.Contains(xattrs, xattrWant)
 
 	if !found {
 		t.Errorf("Flistxattr did not return previously set attribute %q in attributes %v", xattrName, xattrs)