sd: disable pdf import tests

[Stephan Bergmann]

On 17/12/2019 15:29, Ashod Nakashian wrote:
> So these tests aren't really useful anymore. They should be modified to 
> do import+break and then check the results. But that isn't 
> straight-forward and I ran out of time to do it.

In which case it would be great if you could either remove the 
non-useful tests (and I will abandon my 
<https://gerrit.libreoffice.org/#/c/85238/> "Re-enable PDF import 
tests") or document in sd/qa/unit/import-tests.cxx why these tests are 
currently disabled and that the plan is to modify and re-enable them.