`--duration` More precise terminal message when tests with small duration are hidden

Currently when tests are skipped because of the `--duration` flag, a generic message is printed to terminal:

```
(0.00 durations hidden.  Use -vv to show these durations.)
```

I suggest to improve it slightly with the number of hidden tests, as well as precise threshold:

```
(7 items with duration < 0.005s hidden.  Use -vv to show these durations.)
```

What do you think ?
