Switch to aes-256-cbc cipher for Azure account storage (#6412)

* Switch to aes-256-cbc cipher for Azure account storage

* Add unit test

* Add unit test

* Remove unnecessary code

extensions/azurecore/src/test/stubs/credentialsTestProvider.ts

@@ -0,0 +1,33 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as azdata from 'azdata';
+
+/**
+ * Mock CredentialsProvider to be used for testing
+ */
+export class CredentialsTestProvider implements azdata.CredentialProvider {
+	handle: number;
+
+	public storedCredentials: { [K: string]: azdata.Credential } = {};
+
+	saveCredential(credentialId: string, password: string): Thenable<boolean> {
+		this.storedCredentials[credentialId] = {
+			credentialId: credentialId,
+			password: password
+		};
+		return Promise.resolve(true);
+	}
+
+	readCredential(credentialId: string): Thenable<azdata.Credential> {
+		return Promise.resolve(this.storedCredentials[credentialId]);
+	}
+
+	deleteCredential(credentialId: string): Thenable<boolean> {
+		let exists = this.storedCredentials[credentialId] !== undefined;
+		delete this.storedCredentials[credentialId];
+		return Promise.resolve(exists);
+	}
+}