Test every plasmoid

Summary:
ShellCorona will now abort in test mode if an applet fails to load, even
without a dedicated script, this test will make sure everything at least
loads; hopefully preventing the task-manager bug of 5.8.8 and the quickshare
bug of 5.10.0

Simplify plasmoid-test loading.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6134