Now that the mscrypto part of PDFDocument::ValidateSignature() is
implemented it's possible to run these tests on Windows as well,
provided the machine has at least one signing certificate installed.

Also fix a race, where the workdir of the signing test was used by the
pdfsigning test.

Change-Id: I80bbfbb5dc4baa400f9a6b85961883a247b0f22b