Test runner does not work for leap exercise

I've tried running the test runner for the leap exercise (using the example as the source), but I get an error message:

```
leap: testing...
* Reading spec file: /solution/spec/leap_spec.cr 📖
Unhandled exception: Index out of bounds (IndexError)
from /usr/share/crystal/src/array.cr:1363:13 in '__crystal_main'
from /usr/share/crystal/src/crystal/main.cr:110:5 in 'main'
from ???
* Reading scaffold json 📖
Unhandled exception: Error opening file with mode 'r': '/output/scaffold.json': No such file or directory (File::NotFoundError)
from /usr/share/crystal/src/crystal/system/file.cr:7:7 in 'new'
from /usr/share/crystal/src/file.cr:627:12 in 'read'
from helpers/result_to_json.cr:121:30 in '__crystal_main'
from /usr/share/crystal/src/crystal/main.cr:110:5 in 'main'
from ???
leap: done
```

It works fine for `anagram` and `hello-world`.