Add removeItem to localStorage mock in docs (#5919)

Author: phacks

docusaurus/docs/running-tests.md

@@ -239,6 +239,7 @@ For example:
 const localStorageMock = {
   getItem: jest.fn(),
   setItem: jest.fn(),
+  removeItem: jest.fn(),
   clear: jest.fn(),
 };
 global.localStorage = localStorageMock;