Add more test coverage for checkstyle

[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   39.70m ± 5%   39.66m ± 1%       ~ (p=0.315 n=10)
CheckRules-4     8.503 ± 2%    8.521 ± 2%       ~ (p=0.481 n=10)
geomean         581.0m        581.3m       +0.05%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.143 n=10)
CheckRules-4    977.5Mi ± 0%   977.0Mi ± 0%       ~ (p=0.280 n=10)
geomean         163.7Mi        163.6Mi       -0.02%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%  +0.00% (p=0.032 n=10)
CheckRules-4    14.19M ± 0%   14.19M ± 0%       ~ (p=0.247 n=10)
geomean         1.368M        1.368M       -0.01%

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 95.13%. Comparing base (f01f208) to head (f9e586f).

Files with missing lines	Patch %	Lines
internal/reporter/checkstyle.go	66.66%	5 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1145 +/- ## ========================================== + Coverage 95.01% 95.13% +0.11% ========================================== Files 103 103 Lines 10355 10350 -5 ========================================== + Hits 9839 9846 +7 + Misses 360 351 -9 + Partials 156 153 -3 ```

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