tests: Move some functions into a common module.

* tests/openpgp/tofu.scm (gettrust): Moved to the common defs.scm
module.
(checktrust): Likewise.
* tests/openpgp/defs.scm (gettrust): New function.
(checktrust): Likewise.

These functions will be re-used by the tests for the PGP trust
model.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>