Web IDE Frontend Integration Tests in Jest

Context

#202243 (comment 298143742)

Description

Each FE integration test will:

• Mount the application component.
• Walk through an entire use case.
• At key points, take a snapshot of the difference between the last snapshot.

This will complement our unit tests and feature specs nicely by testing all of the FE components in integration in a faster way. We'll also use this higher-level test to identify unit test gaps.

See #202243 (closed)

Proposal

• Let's setup a feature test environment with appropriate mocks.
• Let's write a test for adding and committing a file to a sub directory.

This issue is mostly focused on setting up the necessary test helpers. We'll expand with more tests in the future.