qmltc: test property value assignments and type conversions

(Splitted from QTBUG-91956)

• (in general) support conversions between types; complex cases use QQmlStringConverters (TODO: think whether this is actually in scope of qmltc or in scope of qmlsc/qmlcachegen)
• (in general) must follow QQmlObjectCreator::setPropertyValue() logic
• (at least requires more tests?) (in general) must be aware of type conversions (e.g. bool -> int, int -> bool, etc.)