clarify test

errsizedgroup_test.go

@@ -134,18 +134,17 @@ func TestErrorSizedGroup_TermOnErr(t *testing.T) {
 	for i := 0; i < N; i++ {
 		i := i
 		ewg.Go(func(ctx context.Context) error {
-			val := atomic.AddUint32(&c, 1)
-			if i == errIndex || val > uint32(errIndex+1) {
+			if val := atomic.AddUint32(&c, 1); i == errIndex || val > errIndex+1 {
 				return fmt.Errorf("err from function %d", i)
 			}
 			return nil
 		})
 	}
 
-	err := ewg.Wait()
-
+	err := ewg.Wait() // with some probability it can return "{err from function 101}"
+	require.Len(t, err.(*MultiError).Errors(), 1)
 	require.NotNil(t, err)
-	require.Contains(t, err.Error(), "err from function 100")
+	require.ErrorContains(t, err, "err from function 100")
 	// we don't know how many routines will be executed before the error, but it should be less than 10
 	require.LessOrEqual(t, c, uint32(errIndex+100), fmt.Sprintf("%d, routines have to be terminated early", c))
 }