Update Tests

[kboshold]

To be able to update to the latest version of Weasyprint the tests have to be updated and changed from a PNG comparison to a PDF comparison. It is important to note that the PDF comparison will only compare the visual differences, since metadata in the PDF will of course change.

When updating Weasyprint, the Dockerfiles may need to be adjusted, as the newer version of Weasyprint requires a more recent version of libpango.

You can also find a more up-to-date version with some logging output for our memory leak ticket here:
https://github.com/daKomma/weasyprint-rest/tree/bugfix/32