Improve test coverage

[ReubenJ]

_{↩️ Back to the main Herb.jl Hacktoberfest issue}

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Our test coverage in HerbCore is currently ~55% (Codecov). Adding tests in HerbCore will familiarize you with the core data structures the library builds upon. We would greatly appreciate your help! 🤗

Suggested steps:

1. If you're new to Herb, please take a look at our getting started guide and CONTRIBUTING.md.
2. Check the current test coverage on Codecov.
3. Following the CONTRIBUTING.md guide, set up your own fork of HerbCore and take a look at the existing tests.
4. Once you've added some new tests, open a PR to check that they pass in CI and to double-check that you've increased coverage in the way you were hoping.

Don't hesitate to reach out here for any clarifications. Happy hacking!

[chhote-mote-log]

/assign