Enhance testing framework regards 'selection range' testing

[Garry Yao]

1. Document content along with selection testing regards various command execution result could be simplified as comparison based on our widely used selection description language, which make test case easy to write and understand by people, so a translation API could be provided to make the editor understand this language.

• represent single selection

<p>
	This is some <strong>[sample text]</strong>.</p>

• represent multiple selection

<table>
	<tbody>
		<tr>
			<td>
				Text1
			</td>
			<td>
				[Text2]
			</td>
		</tr>
		<tr>
			<td>
				[Text3]
			</td>
			<td>
				Text4
			</td>
		</tr>
	</tbody>
</table>
<br />

• represent collapsed selection

  	This is some <strong>sample^ text</strong></p>
  

2. Secondly, in order to avoid embed html string in JavaScript codes, it's more convenient to utilize a <textarea> to hold the actually expected/provided descriptive HTML codes( the reason for bypass <pre> is because IE's unable to preserve original format when retrieving content dynamically). So a cross-browser implementation could be established to get pre-formatted content with ease.

[Garry Yao]

Proposed selection test sample.

[Garry Yao]

This function should be provided as an CKEDITOR.test API.

[Garry Yao]

Design TC added at : ​http://ckeditor.t/dt/core/test.html.

[Garry Yao]

Changes committed with #4144, leave the ticket open for collecting feebacks.