backend: Improve test for pagination in list

internal/backend/test/tests.go

@@ -11,6 +11,7 @@ import (
 	"reflect"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -250,7 +251,21 @@ func (s *Suite) TestList(t *testing.T) {
 	const numTestFiles = 1233
 	list1 := restic.NewIDSet()
 
+	var wg sync.WaitGroup
+	input := make(chan int, numTestFiles)
 	for i := 0; i < numTestFiles; i++ {
+		input <- i
+	}
+	close(input)
+
+	output := make(chan restic.ID, numTestFiles)
+
+	for worker := 0; worker < 5; worker++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+
+			for i := range input {
 				data := []byte(fmt.Sprintf("random test blob %v", i))
 				id := restic.Hash(data)
 				h := restic.Handle{Type: restic.DataFile, Name: id.String()}
@@ -258,6 +273,15 @@ func (s *Suite) TestList(t *testing.T) {
 				if err != nil {
 					t.Fatal(err)
 				}
+				output <- id
+			}
+		}()
+	}
+
+	wg.Wait()
+	close(output)
+
+	for id := range output {
 		list1.Insert(id)
 	}