Testing style guide

[mattrosno]

Summary

Currently, our test suite is a mix of enzyme, React Testing Library, and a custom solution with an RTL-like API. We are in need of an official "testing style guide" that maintainers and contributors can reference when authoring or updating components.

Job stories

1. When contributing to Carbon, I want to know how components need to be tested, so I can land my changes.
2. When evaluating and using Carbon, I want to know how components are tested so I can have confidence in test coverage and save time by not maintaining Carbon-level tests within my product's test suite.

Success measures

1. Existing test suite is refactored to use React Testing Library
2. The team's process is documented in the style docs

Sponsoring team

Carbon team

[mattrosno]

Epic: carbon-design-system/carbon#8143