[Tests]Make radiobutton3.qml use PC3

Summary: Correct test radiobutton3.qml to use Plasma Components 3 instead of 2.

Test Plan:
Run qmlscene radiobutton3.qml
Before:
Misaligned button and big height. (bug in PC2 radiobutton)
After:
Correct height and alignment between button and button's text.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: ngraham, davidedmundson, ndavis, kde-frameworks-devel

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D27082