chore: increase code coverage

[JosiahParry]

This PR seeks to improve the overall code coverage of {rextendr} without using snapshot tests.

This will hopefully help us identify any regressions in the future.

[codecov[bot]]

Codecov Report

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

Project coverage is 83.00%. Comparing base (3ee8ae3) to head (4bfeb68).

Additional details and impacted files

[](https://app.codecov.io/gh/extendr/rextendr/pull/405?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=extendr) | [Files with missing lines](https://app.codecov.io/gh/extendr/rextendr/pull/405?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=extendr) | Coverage Δ | | |---|---|---| | [R/cran-compliance.R](https://app.codecov.io/gh/extendr/rextendr/pull/405?src=pr&el=tree&filepath=R%2Fcran-compliance.R&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=extendr#diff-Ui9jcmFuLWNvbXBsaWFuY2UuUg==) | `83.78% <100.00%> (+5.40%)` | :arrow_up: | | [R/use\_msrv.R](https://app.codecov.io/gh/extendr/rextendr/pull/405?src=pr&el=tree&filepath=R%2Fuse_msrv.R&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=extendr#diff-Ui91c2VfbXNydi5S) | `100.00% <100.00%> (+54.54%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/extendr/rextendr/pull/405/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=extendr)

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