Improve readability of sinon assertions by using named spies and sinon-chai

[tomalec]

📝 Provide a description of the improvement

At today's daily standup CKE5 team agreed to:

1. Use sinon-chai, to be able to use expect syntax and custom messages for sinon assertions.

    expect( myMethodSpy ).to.be.calledOnce

2. Use named spies.

   namedSpy = sinon.spy().named( 'myMethod' );
   expect( myMethodSpy ).to.be.calledOnce // expected ~~spy~~ myMethod to be called once but was called twice

Actions to be taken:

• add it and use it in the ckeditor5 - Add sinon-chai to the karma config. ckeditor5-dev#649,
• add a note to the style guide - Add note about named spies and sinon-chai to the code style guide.  #7524,
• (optional) refactor existing assertions.

📃 Other details

• Browser: n/a
• OS: n/a
• CKEditor version: master@17d7bd7390
• Installed CKEditor plugins: n/a

---

If you'd like to see this improvement implemented, add a 👍 reaction to this post.

[tomalec]

We still need the docs entry, don't we?

[tomalec]

@pomek PR for docs is ready #7524