[RFC]: add tests to `@stdlib/dstructs/compact-adjacency-matrix`

### Description

This RFC requests that tests be added to [`@stdlib/dstructs/compact-adjacency-matrix`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/dstructs/compact-adjacency-matrix).

Currently, the package lacks sufficient tests. Ideally, this package would achieve 100% test coverage.

What should be tested?

- All methods
- Property access
- All edge cases

For reference, see [`@stdlib/array/complex64`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/complex64) for a package which exports a custom class and includes extensive unit tests.

### Related Issues

None.

### Questions

No.

### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.

Start in lib/node_modules/@stdlib/dstructs/compact-adjacency-matrix and inspect its exported methods and properties. Use the @stdlib/array/complex64 package as a reference for test structure and coverage. Done means adding tests for all methods, property access, and edge cases, ideally reaching 100% coverage.

Written by the indexing model from the issue text.