Test fails when using RUST_NEW_ERROR_FORMAT

If you set RUST_NEW_ERROR_FORMAT, running the full 'make check' will fail.  @alexcrichton did some investigation and noticed this was one failure: https://github.com/rust-lang/rust/blob/master/src/test/run-make/unicode-input/span_length.rs#L94-L98

We may want to update the test or possibly move it to a UI test.
