ca: Do not expect coverage of right-on-boundary codepath.

Make coverage tests tolerate the no-op code path where the backup ends
right on a block boundary not being exercised.

caucase/ca.py

@@ -864,6 +864,8 @@ class UserCertificateAuthority(CertificateAuthority):
         authenticator.update(buf)
         yield buf
         yield authenticator.finalize()
+      else: # pragma: no cover
+        pass
     for chunk in signingIterator():
       write(encryptor.update(padder.update(chunk)))
     write(encryptor.update(padder.finalize()))
@@ -966,6 +968,8 @@ class UserCertificateAuthority(CertificateAuthority):
         authenticator.update(chunk)
         authenticator.verify(buf[-32:])
         yield chunk
+      else: # pragma: no cover
+        pass
 
     db_class.restore(db_path=db_path, restorator=verificator())