Add PDF comparisons to improve testing

[asgrim]

At the moment we just assert the PDF is generated. We should also do a visual comparison to check the output matches. Note, just doing a binary diff is not sufficient, since the content DOES change every time (even if it is only /CreationDate etc.)

Might be worth taking a look at rst2pdf technique https://github.com/rst2pdf/rst2pdf/blob/master/rst2pdf/tests/compare_pdf.sh for PDF comparison

[ciaranmcnulty]

I wonder if content-length is stable

[Ocramius]

unlikely: could have something as trivial as the name/version of the software generating the PDF baked into it, and it would break.

Visual diffing is something we've done in the past, and it's fairly OK.

[ciaranmcnulty]

The name/version of the software is baked into the image! I guess dates are in there