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.

[petertseng]

I suggest that our track wait until in-flight changes of exercism/problem-specifications#522 (completed at this time) and exercism/problem-specifications#523 (not yet complete at this time) are finished before acting on this.

[petertseng]

I found a way to get around the fact that exercism/problem-specifications#523 was not completed!!!

Just assume that it will be completed in the future, and act as if it is already completed. Great, huh!