Most of these tests used this directive as a way to opt in to a new
language feature, and most of those features are already stable for a
long time, so no opt-in is needed. Some other tests used the directive
to opt out from a language feature, replace those by the `LANGUAGE`
directive. One test used the directive to test behavior that actually
depended on the API version; use `API_VERSION` directive there instead.
Enum entries are "special" kind of singletons that should be
referenced as a captured 'this' instance inside during entry
initialization, because corresponding static fields in enum class
are not initialized yet.
#KT-7257 Fixed
Mikhail Glukhikh