Test text insertion before <picture>.

Initial state:
| <picture>
|   <source>
|     media="(min-width: 600px)"
|     srcset="resources/apple.gif"
|   <source>
|     srcset="resources/mozilla.gif"
|   <#selection-caret>
|   <img>
|     src=""

After insertion:
| "text<#selection-caret>"
| <picture>
|   <source>
|     media="(min-width: 600px)"
|     srcset="resources/apple.gif"
|   <source>
|     srcset="resources/mozilla.gif"
|   <img>
|     src=""