feat: run snapshot tests dynamically in unimarkup-core crate

[nfejzic]

This PR adds dynamic testing of snapshots in unimarkup_core crate.

By dynamic is meant that all spec files are automatically collected, parsed in order to generate test cases, and these test cases are run. Snapshot files are automatically created, and each test case (where each spec yaml file can contain multiple tests) is reported in cargo test output with corresponding name (and module path).

Downside of using custom harness is that using #[test] attribute to automatically test functions does not work (as far as I know). Any custom function must be manually collected. See test_fn! macro and how it's used.

Closes #100.

[mhatzl]

Thanks for this PR.

The blocks.rs file in core seems to be missing the snapshot test runner.
At the moment, only spec tests are run if I am correct.

Everything else looks ready to be merged.

[mhatzl]

Looks good now.
Just not sure about case2 as variable name.

But I can change that myself once I merge #106 with these changes.
There will be some merge conflicts anyways, because of the Scanner change.

[mhatzl]

thanks 😊
looks good now 👍