tests: Minor imporvement in agent invocation

* tests/openpgp/defs.scm (create-gpghome): Add s2k-count.

My tests show only 2.5% improvement, but as we have that option now
let's use it.

real 9m12.604s
user 2m20.720s
sys 0m11.452s

real 8m3.815s
user 2m16.700s
sys 0m11.544s

• Signed-off-by: Werner Koch <wk@gnupg.org>