Changes to Luhn tests

[IanWhitney]

We recently rewrote the test suite for the Luhn exercise. Since this track implements Luhn, please take a look at the new canonical_data.json file and see if your track should update its implementation or tests.

Details on the test changes and the motivation can be found in the PR.

Tracking of these changes is being done at the tracking issue.

[rchavarria]

(same as xjavascript)

I fear the specs here are quite different from the canonical data and we should reimplement both specs and example implementation 😢