Add an autotest for ActionToolBar

Summary:
This adds an inital test to test whether ActionToolBar does the
right thing. It does not yet cover display hints or custom
components, but does cover using QQC2 action instead of Kirigami
Action.

Test Plan: The new test passes.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27700