Use -Xlambdas=class in tests which were checking specific things related
to how anonymous classes for lambdas work (such as receiver mangling,
function arity etc.)
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.
Alexander Udalov