DictionaryComboBoxTest: Add stretch to avoid expanding Dump button

Test Plan:
Original: Note the ellipsis in the middle. A reason to expand the window

With and without patch

Sorry to bother you with this pettiness.

Reviewers: Frameworks, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17026