The build system l3build normally writes the contents of some boxes from .lvt files into corresponding .tlg files. Sometimes a dependent package adds e.g. the command \kern0pt, so that test files fail, even if the PDF files look the same as before and are still correct. The ppmcheckpdf tool offers an alternative option for regression testing: instead of printing the contents of boxes in .lvt files, PDF files are converted to PNG files and you can compare the PNG files after l3build has finished its work.