Add CI tests for the new card-backend.

• Refactor existing tests to allow running most tests again both the softkey and card backend
  • Use "features" mechanism to define which tests to run (card tests can only be run when there is a [virtual] card to run them against)
• Add tests/card.rs for new, card-specific integration tests
• Fold gnupg-test-wrapper code back into the openpgp-ca crates (causes duplication, but simplifies debian packaging)