Fix SSL tests on 32-bit Perl

Fix SSL tests on 32-bit Perl

The certificate serial number generation was changed in b4c4a00ea to
use the current timestamp. The testharness must thus interrogate the
cert for the serialnumber using "openssl x509" which emits the serial
in hex format. Converting the serial to integer format to match whats
in pg_stat_ssl requires a 64-bit capable Perl. This adds a fallback
to checking for an integer when the tests with a 32-bit Perl.

Per failure on buildfarm member prairiedog.

Discussion: https://postgr.es/m/0D295F43-806D-4B3F-AB98-F941A19E0271@yesql.se