Disallow `fs` in integration specs

• Step 1 - Handle entry not found in changeFileContent action
• Step 2 - Disallow fs in integration specs
• Step 3 - Resolve "Improve slow tests in ./spec/features/ide/user_sees_editor_info_spec.rb"

What does this MR do?

One of the benefits to using miragejs in our FE integration spec environment is that you can run it in the browser:

https://youtu.be/nQIHzyl9AZ4?t=1322

This isn't possible if we import fs as a module. This MR does some refactoring so that we can import 'test_helpers/mock_server/use'; in production code during development.