Add support to test printing pdfs

[calixteman]

It's possible with puppeteer to print into a pdf:
https://github.com/puppeteer/puppeteer/blob/main/docs/api.md#pagepdfoptions

So we could load a pdf, dump it into another pdf (if @media print stuff is ok), then load the dumped one and finally generate a PNG.

[marco-c]

Once we have this capability, we should write a regression test for #13852.

[marco-c]

This is pretty important, we've been discovering lots of bugs in "Save to PDF" (see for example https://bugzilla.mozilla.org/show_bug.cgi?id=1777209) and the test coverage in m-c is lacking.

[marco-c]

As part of this, we shouldn't only compare the rendering, but also ensure the PDF output is not rasterized.