Use correctly rendered pdf for webpage-spec-renders tests

The last reference test.pdf in the tests was using a complete different region
of the input "image.jpg" because the used phantomjs version used to generate it
had a bug. This bug was fixed in 833eb824f5bae07f1b157d6757e7e3c98343f812
("Don't scale the unit px to 1/2.54 points for PDFs") and
1daa2eb4dd49efb848ff96e37f298a774520cc9b ("Disable page shrinking for pdf
printing to create accurate output").

The new version should now use the same region of the input image as the other
generated files for the gif, jpg and png tests. The only difference is the
extra height is still displayed on a second page but this is currently expected
by phantomjs.

https://github.com/ariya/phantomjs/issues/11590 ("page.paperSize is not
accurate for .pdf")