Fix saving of PNG images for failed tests.
Fix saving of PNG images for failed tests.

andrewleech/WeasyPrint,prepare/TestWeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint,marclaporte/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,Kozea/WeasyPrint,marclaporte/WeasyPrint,andrewleech/WeasyPrint