tests: Fix convenience function to run gpg.

* tests/openpgp/defs.scm (pipe:gpg): Remove stray dash.

The extra dash is problematic at export and import because this does
not export a single key as intended by the tests but does two key
lookups where the second fails because that userid is always "-". For
the current keyring and keybox code this does not matter because it is
ignored. The keyboxd code however is more picky and returns an error.

• Fixes-commit: 9609cb20e4caee739b9fc4fd36797029d2970041
• Signed-off-by: Werner Koch <wk@gnupg.org>