Fix tests with -DWITH_POPPLER=OFF

New poppler releases often break API so an older versions of Inkscape might not build with latest system packages. That makes bisecting quite annoying – one either needs to backport Inkscape patches from master or downgrade poppler package. So when not debugging PDF input, I want to disable Poppler support. However, that previously broke some tests.

Let’s make tests of features that depend on poppler conditional on building Inkscape with Poppler support.