This test was ignored long ago in
https://github.com/rust-lang/rust/pull/20578/ when the syntax for
closures was changed.
The current status is that a closure with an explicit `!` return type
will trigger the `unreachable_code` lint which appears to be the
original intent of the test
(https://github.com/rust-lang/rust/pull/16836). A closure without a
return type won't trigger the lint since the `!` type isn't inferred
(AFAIK). This restores the test to its original form.
