Back-story: After reflection this morning, I realized that the
previous form of this test would allow the macro invocation to treat
the `dyn` input as a raw-identifier rather than a keyword, and since
the input was discarded by that version of the macro, the test would
pass despite the detail that the input `dyn` should not have been
parsed as a raw-identifier.
This revision fixes that oversight, by actually *using* the macro
input to construct a `Box<dyn Trait>` type.
