Move test-only function from cffi/ to testing/

Was already merged in Bitbucket before import, marked as merged by the import user

This avoids having the cffi package trying to import from testing, even though the latter is only available during testing and not after installation.

It also fixes an ImportError when running pypy3's cffi tests. Note that the same tests only worked on pypy2 due to from testing.udir import udir automagically morphing from an absolute import into an implicit relative one, and to the existence of the spurious cffi.testing subpackage.