Test no incorrect 'data:image/png;base64,b' in html files.

Check  that HTML conversion of a notebook with images by "nbconvert --to html" is ok.

It currently fails under python3 (detected in issue #3903 )

bad transformation =
 img src="data:image/png;base64,b'iVBOR....

good transformation = 
img src="data:image/png;base64,iVBOR....