[test] improve segment explorer tests snapshot (#81616)
Format line format: `<segment> [<files,>]`
This will help visualize the rendered segment explorer panel easier by checking the snapshot. e.g.
```
app/ [layout.tsx]
parallel-routes/ [layout.tsx, page.tsx]
@bar/ [layout.tsx, page.tsx]
@foo/ [layout.tsx, page.tsx]
```
