Added digit 9 test case from the canonical test suite

9 is an edge case that is very important to keep testing in the advanced Luhn iteration exercises (Luhn From and Luhn Trait).

I know it because I was optimizing my solutions and after passing all the From and Trait tests, when I put those optimizations in my original Luhn solution, it failed the Luhn tests. Turns out the canonical specification had a test case for this! So I’m adding it to the advanced exercises as well.

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