iq/8/sudoku: bring test coverage from 62.7% to 80%

```console
% check-go-test -cover 80 `find ./iq/8/sudoku -type f -name \*.go`
=== COVERAGE: github.com/skhal/lab/iq/8/sudoku
    coverage: 62.7% of statements
    threshold: 80.0%
--- FAIL
```