chore(docs): Adds clearMocks to unit-testing

[AbbottSoft]

Description

Updates unit-testing.md to include the clearMocks property in jest.config.js.

The instructions in step #3 suggest setting up a globally mocked gatsby object in the __mocks__ directory at the root of the project. However, if anyone were to utilize these mocks, such as mocking the return value in a test, the state of the mocks would persist between tests, leading to unexpected outcomes. This change attempts to address this by adding the clearMocks configuration to the jest config so that developers following this tutorial will not need to manually reset the mocks on each test.

Documentation

Jest clearMocks documentation

Related Issues

Issue #26795 - Mocking gatsby in mocks not cleaning up after tests

[LekoArts]

Hey! Thanks so much for opening a pull request!

The change you’ve proposed is not going to be accepted because this change is a question of preference and in those cases we want to stick to the defaults unless we have a specific reason not to. A lot of people (including me) choose to explicitly clear the mocks via jest.clearAllMocks() in the afterEach() hook. Feel free to change your jest-config file however we'll want to stick to the default here.

We absolutely want to have you as a contributor, so please take a look at our open issues for ideas, and please reach out to us on Twitter at @gatsbyjs with questions.

Thanks again, and we look forward to seeing more PRs from you in the future! 💪💜