tests: Speed up conventional encryption tests for gpg.

* tests/openpgp/conventional-mdc.test: Add an s2k-count option.
* tests/openpgp/conventional.test: Ditto.

Due to measuring the iteration count for the passphrase hashing, the
conventional encryption tests are running quite slow. This patch
fixes it by using a fixed and lower value for the iteration count.

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