Introduce KeyStore protocol in order to provide specialized key store implementations for tests

With the DictBasedKeychain the main AppKeychain is not influenced by tests. The previous implementation led to an empty Keychain requiring a new setup of the simulator.

passKit/Helpers/KeyStore.swift

@@ -0,0 +1,19 @@
+//
+//  KeyStore.swift
+//  passKit
+//
+//  Created by Danny Moesch on 20.07.19.
+//  Copyright © 2019 Bob Sun. All rights reserved.
+//
+
+import Foundation
+
+public protocol KeyStore {
+    func add(data: Data?, for key: String)
+    func add(string: String?, for key: String)
+    func contains(key: String) -> Bool
+    func get(for key: String) -> Data?
+    func get(for key: String) -> String?
+    func removeContent(for key: String)
+    func removeAllContent()
+}