Add more test coverage

[github-actions[bot]]

Benchmark diff:

goos: linux
goarch: amd64
pkg: github.com/cloudflare/pint/cmd/pint
cpu: AMD EPYC 7763 64-Core Processor                
              │   old.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
FindEntries-4   40.84m ± 2%   40.68m ± 4%       ~ (p=0.631 n=10)
CheckRules-4     8.511 ± 2%    8.514 ± 2%       ~ (p=0.631 n=10)
geomean         589.6m        588.5m       -0.18%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.089 n=10)
CheckRules-4    984.5Mi ± 0%   983.7Mi ± 0%       ~ (p=0.063 n=10)
geomean         164.3Mi        164.2Mi       -0.04%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.897 n=10)
CheckRules-4    14.28M ± 0%   14.28M ± 0%  -0.03% (p=0.011 n=10)
geomean         1.372M        1.372M       -0.02%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.80%. Comparing base (e233a20) to head (8cd062f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1165 +/- ## ======================================= Coverage 94.80% 94.80% ======================================= Files 103 103 Lines 12166 12168 +2 ======================================= + Hits 11534 11536 +2 Misses 482 482 Partials 150 150 ```

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