[ Reason ]
Fix FTBFS bug #1102313.

[ Impact ]
Not everybody will be able to build the package.

[ Tests ]
I've checked that the package builds ok again on the systems where it used to fail.

[ Risks ]
Very low. We are just conditionally skipping a test when we know that it will fail
(i.e. on single-cpu systems).

[ Checklist ]
   [X] *all* changes are documented in the d/changelog
   [X] I reviewed all changes and I approve them
   [X] attach debdiff against the package in (old)stable
   [X] the issue is verified as fixed in unstable

[ Changes ]
See changelog.

[ Other info ]
Change is trivial. I will upload the package next.