fix Hints CB test

op-program/preimage/hints_test.go

@@ -87,20 +87,30 @@ func TestHints(t *testing.T) {
 		require.ErrorIs(t, err, io.ErrUnexpectedEOF)
 	})
 	t.Run("cb error", func(t *testing.T) {
-		var buf bytes.Buffer
-		hw := NewHintWriter(&buf)
-		hw.Hint(rawHint("one"))
-		hw.Hint(rawHint("two"))
-		hr := NewHintReader(&buf)
-		cbErr := errors.New("fail")
-		err := hr.NextHint(func(hint string) error { return cbErr })
-		require.ErrorIs(t, err, cbErr)
-		var readHint string
-		err = hr.NextHint(func(hint string) error {
-			readHint = hint
-			return nil
-		})
-		require.NoError(t, err)
-		require.Equal(t, readHint, "two")
+		a, b := bidirectionalPipe()
+		var wg sync.WaitGroup
+		wg.Add(2)
+
+		go func() {
+			hw := NewHintWriter(a)
+			hw.Hint(rawHint("one"))
+			hw.Hint(rawHint("two"))
+			wg.Done()
+		}()
+		go func() {
+			defer wg.Done()
+			hr := NewHintReader(b)
+			cbErr := errors.New("fail")
+			err := hr.NextHint(func(hint string) error { return cbErr })
+			require.ErrorIs(t, err, cbErr)
+			var readHint string
+			err = hr.NextHint(func(hint string) error {
+				readHint = hint
+				return nil
+			})
+			require.NoError(t, err)
+			require.Equal(t, readHint, "two")
+		}()
+		wg.Wait()
 	})
 }