test failure on 32 bit systems

With dbab8a01 I broke the certificate-generation.scm test on systems where a long is only 32 bits.

Also there is currently the assumption that long and time_t always have the same size which may or may not hold true.

In !33 (merged) I shared a snippet to use the right size int for the conversion but I don't know how I would make the test less demanding when time_t is only 32 bits.