This tests ensures formatBlock removes a pre when formatting multiple paragraphs inside the pre.

Formatting all paragraphs by h3 yields:
| "
"
| <h3>
|   "<#selection-anchor>hello"
|   <br>
|   "
"
|   "world"
|   <br>
|   "
"
|   "webkit<#selection-focus>"
| "
"

Undo yields:
| "
"
| <pre>
|   "<#selection-anchor>hello

world

webkit<#selection-focus>
"
| "
"

Formatting all but the last paragraph by h3 yields:
| "
"
| <pre>
|   <h3>
|     "<#selection-anchor>hello"
|     "
"
|     "world"
|     "
"
|   "<#selection-focus>webkit
"
| "
"

Undo yields:
| "
"
| <pre>
|   "<#selection-anchor>hello

world

<#selection-focus>webkit
"
| "
"

Formatting all but the first paragraph by h3 yields:
| "
"
| <pre>
|   "hello
"
|   <h3>
|     "<#selection-anchor>
"
|     "world"
|     "
"
|     "webkit<#selection-focus>"
| "
"