add detailed API tests checking how calls to PGPAgent propagate to the underlying interface

this is refactoring support, so that we can notice changes in how the
underlying APIs are called, and make changes intentionally when needed,
instead of accidentally.

passKit/Crypto/PGPAgent.swift

@@ -17,6 +17,11 @@ public class PGPAgent {
         self.keyStore = keyStore
     }
 
+    init(keyStore: KeyStore, pgpInterface: PGPInterface) {
+        self.keyStore = keyStore
+        self.pgpInterface = pgpInterface
+    }
+
     public func initKeys() throws {
         guard let publicKey: String = keyStore.get(for: PGPKey.PUBLIC.getKeychainKey()),
               let privateKey: String = keyStore.get(for: PGPKey.PRIVATE.getKeychainKey()) else {