Make component unit test for editor/components/warning/index.js

[hideokamoto]

Description

Related to progress on the issue -> Unit Testing Components #641

Added some unit test scripts for Warning component.

How Has This Been Tested?

Run unit test command.

$ npm test

 PASS  editor/components/warning/test/index.js
  Warning
    ✓ should match snapshot (16ms)
    ✓ should has valid class (2ms)
    ✓ should show child error message element (6ms)

----------------------------------------------------|----------|----------|----------|----------|----------------|
File                                                |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------------------------------------------------|----------|----------|----------|----------|----------------|
editor/components/warning                          |      100 |      100 |      100 |      100 |                |
 index.js                                          |      100 |      100 |      100 |      100 |                |

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[gziolo]

This looks great, thanks for your contribution! 👍