iq/15/lru: bring test coverage from 77.0% to 80%

```console
% check-go-test -cover 80 `find ./iq/15/lru -type f -name \*.go`
=== COVERAGE: github.com/skhal/lab/iq/15/lru
    coverage: 77.0% of statements
    threshold: 80.0%
--- FAIL
```