chore: Add some basic encoder tests

[nmunnich]

Adds some very basic encoder tests for rotating, and basic layers. Mock Kscans don't seem to interact nicely, slightly on the hacky side. However, better to have hacky rudimentary tests than no tests.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.