fix Hints CB test

op-program/preimage/hints_test.go

@@ -87,11 +87,19 @@ 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)
+		a, b := bidirectionalPipe()
+		var wg sync.WaitGroup
+		wg.Add(2)
+
+		go func() {
+			hw := NewHintWriter(a)
 			hw.Hint(rawHint("one"))
 			hw.Hint(rawHint("two"))
-		hr := NewHintReader(&buf)
+			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)
@@ -102,5 +110,7 @@ func TestHints(t *testing.T) {
 			})
 			require.NoError(t, err)
 			require.Equal(t, readHint, "two")
+		}()
+		wg.Wait()
 	})
 }