Add unit test for pack pref-dim bug
user
```factor
USING: accessors tools.test ui.gadgets ui.gadgets.labels ;
IN: ui.gadgets.labels.tests

[ { 119 14 } ] [
    <gadget> { 100 14 } >>dim
    <gadget> { 14 14 } >>dim
    label-on-right { 5 5 } >>gap
    pref-dim
] unit-test
```
assistant