refactor: use slices.Contains to simplify code (#10121)

There is a [new function](https://pkg.go.dev/slices@go1.21.0#Contains)
added in the go1.21 standard library, which can make the code more
concise and easy to read.

lib/model/folder_test.go

@@ -8,6 +8,7 @@ package model
 
 import (
 	"path/filepath"
+	"slices"
 	"testing"
 
 	"github.com/d4l3k/messagediff"
@@ -117,20 +118,11 @@ func unifySubsCases() []unifySubsCase {
 	return cases
 }
 
-func unifyExists(f string, tc unifySubsCase) bool {
-	for _, e := range tc.exists {
-		if f == e {
-			return true
-		}
-	}
-	return false
-}
-
 func TestUnifySubs(t *testing.T) {
 	cases := unifySubsCases()
 	for i, tc := range cases {
 		exists := func(f string) bool {
-			return unifyExists(f, tc)
+			return slices.Contains(tc.exists, f)
 		}
 		out := unifySubs(tc.in, exists)
 		if diff, equal := messagediff.PrettyDiff(tc.out, out); !equal {
@@ -146,7 +138,7 @@ func BenchmarkUnifySubs(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		for _, tc := range cases {
 			exists := func(f string) bool {
-				return unifyExists(f, tc)
+				return slices.Contains(tc.exists, f)
 			}
 			unifySubs(tc.in, exists)
 		}