test: improve test coverage

[codecov[bot]]

Codecov Report

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

Project coverage is 94.32%. Comparing base (f0687a6) to head (451819b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #73 +/- ## ========================================== + Coverage 92.75% 94.32% +1.57% ========================================== Files 54 54 Lines 5338 5497 +159 ========================================== + Hits 4951 5185 +234 + Misses 387 312 -75 ```

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Bencher

Report	Sun, March 24, 2024 at 18:34:30 UTC
Project	theseus-rs/rsql
Branch	test-coverage
Testbed	localhost

Benchmark	Latency	Latency Results nanoseconds (ns)
postgresql	➖ (view plot)	796130000.000
sqlite	➖ (view plot)	251080.000

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