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.

passKitTests/Testbase/DictBasedKeychain.swift

@@ -0,0 +1,42 @@
+//
+//  DictBasedKeychain.swift
+//  passKitTests
+//
+//  Created by Danny Moesch on 20.07.19.
+//  Copyright © 2019 Bob Sun. All rights reserved.
+//
+
+import Foundation
+import passKit
+
+class DictBasedKeychain: KeyStore {
+    private var store: [String: Any] = [:]
+
+    public func add(data: Data?, for key: String) {
+        store[key] = data
+    }
+
+    public func add(string: String?, for key: String) {
+        store[key] = string
+    }
+
+    public func contains(key: String) -> Bool {
+        return store[key] != nil
+    }
+
+    public func get(for key: String) -> Data? {
+        return store[key] as? Data
+    }
+
+    public func get(for key: String) -> String? {
+        return store[key] as? String
+    }
+
+    public func removeContent(for key: String) {
+        store.removeValue(forKey: key)
+    }
+
+    public func removeAllContent() {
+        store.removeAll()
+    }
+}