Improve test coverage

[pwochner]

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

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This repository currently has 50% test coverage. Adding tests to HerbSpecification.jl will familiarize you with the way program synthesis problems are specified in Herb.jl. Currently, only the problem specifications in the form of input-output examples are supported. We would greatly appreciate your help!

Suggested steps:

1. If you're new to Herb, please take a look at our main Herb.jl Hacktoberfest issue for some helpful resources to get you started.
2. Check the current test coverage on Codecov.
3. Following the CONTRIBUTING.md guide, set up your own fork of HerbSpecification 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!