Various fixes to make autotests run and pass

Summary:
Apparently Kirigami's autotests have not run for a while. This
patch fixes that and makes some other changes so the tests pass
again and in the future it should be easier to add tests.

Changes:

• Move add_subdirectory(autotests) to a location where BUILD_TESTING is defined so we actually can add the dir.
• Add each test QML file as a separate test so reports are generated per test and tests can be run individually.
• Copy the plugin .so file to bin/ so autotests can acually import Kirigami.
• Add an autotest to test creation and rendering of Icon.
• Fix most of the existing tests by making sure the window they rely on actually gets shown.
• Fix PageRow test by changing it to matching current behaviour of PageRow.

Test Plan: With this, "make test" should work and pass.

Reviewers: Kirigami, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27521