We must be able to create the key files.

As described the previous commit:
 - 5a5bff2f

We must use this method to create the required files to test if the
signature is working as expected.

So, first we create the files...

src/zc/buildout/networkcache.txt

@@ -3,6 +3,8 @@ Using buildout with cache over network
 
    >>> nc_data = tmpdir('nc_data')
    >>> nc_url = start_nc(nc_data)
+   >>> signature_data = tmpdir('signature_data')
+   >>> public_certificate_path, private_key_path = create_signature_file_list(signature_data)
 
 Networkcache in buildout is optional, and during normal run no information
 is shown: