From the description of the inspection:
When using libraries that define their own global symbols outside
their visible JavaScript code (e.g. describe() in Mocha), it is
recommended that you add the corresponding TypeScript type definition
file as a JavaScript library in Preferences | Languages & Frameworks |
JavaScript | Libraries.
Navigate to the above referenced preference and make sure the @types/mocha (this was changed in early 2019 from mocha-DefinitelyTyped. Either is OK.) library is in the list (don't select it). If it is not in the list, click the Download... button, find mocha there, and add it. Then click the Manage Scopes... button, find your test directory, click on the library column, and select the @types/mocha entry. See the screenshot below.
