kcm/SpeakerTest: Fix the minimum layout for Mono channel
The label needs to set fillWidth: true to influence the sheet's
minimum width, so it doesn't get cut out.
This is only noticeable when the grid layout is smaller, when we
have single Mono channel. Also slightly adjust the Grid's sizes
and alignment for that case
| BEFORE | AFTER |
|---|---|
 |
 |
BUG: 476096
FIXED-IN: 6.1