tests: Enable gpgconf test.
* tests/openpgp/Makefile.am (XTESTS): Re-add gpgconf.scm.
This partially reverts 25e029823813e190a18b601af60efcb1fb3b84af.
That change was not sufficiently motivated, and our continuous
integration server shows no recent distcheck failures.
- Signed-off-by: Justus Winter <justus@g10code.com>