[autotests] Unit most scripted effects API

Summary:
Ready for QJSEngine port and upcoming other fixes.
Split as it makes it easier to do any before/after testing.

Test Plan:
All tests pass with the current QScriptEngine
Verified expected API against a wiki page and current code.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14482