Introduce `unitt`

[RickBarretto]

Description

Implement some tests. Right now, this is just an experiment, and I don't plan to convert everything in a single PR, but in multiple ones.

The idea is to increase the readability, reduce indeterminism and revise the current implementation of libraries and others. Also, tests should work as a kind-of reference book for the users and other developers.

Some principles:

• Tests must be readable
• Tests must be independent from each other
• Skipped tests must be explicitly skipped
• Tests must run quickly
• Tests must be the simply enough to guarantee confidence when refactoring
  • The plan is not to write tests itself, but earn confidence and stability

Type of change

• [x] Code cleanup
• [x] Unit tests (added or updated unit-tests)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Enhancement (implementation update, or general performance enhancements)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (documentation-related additions)

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Reference

See RickBarretto\unitt