Summary

continuing #150

When writing a library that might be consumed by other libraries, how can we best provide mocks for those downstream project? In other words, given a library called font which depends on one called pixel, what should pixel look like? Initial considerations are the following:

• Some ability for pixel to provide a mock class (or other testing aid) that font can have access to
• Some ability to easily switch between mocks and real classes (support dependency injection?)
• Assurance that there is no cost (in compiled bytes on real hardware) for providing test classes. (Compiler flags?)
• Ability to test pixel mocks from pixel CI
• How to structure test fixtures for discoverability
• etc