waitForImage should call callback if image is already loaded

[engineering-this]

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Test tool waitForImage calls callback on load event, but when image is already loaded this event will never happen, and some browsers might load images earlier than others.

https://github.com/ckeditor/ckeditor-dev/blob/master/tests/plugins/uploadfile/_helpers/waitForImage.js

I propose to add if branch:

else if ( image.$.complete )  {
    callback()
}

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• Browser: …
• OS: …
• CKEditor version: …
• Installed CKEditor plugins: …

[mlewand]

To be consistent the callback should be called asynchronously - other than that I think it's an ok enhancement.