Introduce assertEqualMarkup() test util

[jodator]

Suggested merge commit message (convention)

Feature: Introduce assertEqualMarkup() test util method. Closes ckeditor/ckeditor5#2514.

---

Additional information

• I'll check if we can use it somewhere else other then PFO - tables?
• 2 New dev dependencies:
  • 1. js-beautify for formatting HTML code: https://www.npmjs.com/package/js-beautify
  • 2. assertion-error for throwing chai compatible assertion errors (already added by chai from the testing environment).

Required by ckeditor/ckeditor5-engine#1779

[coveralls]

Coverage remained the same at 99.828% when pulling c0c7d5f on t/ckeditor5-paste-from-office/14 into 36db591 on master.

[Reinmar]

2 New dev dependencies:
*

Interestingly, we already used both because I didn't have to install anything.

[Reinmar]

This looks really cool:

[Reinmar]

The trick used to replace <$text> elements is a bit ugly (it creates invalid HTML because we also need to take care of closing tags). But if it works, it's fine.