Fixed static tests and backported unit test

app/code/Magento/Customer/Model/Plugin/CustomerFlushFormKey.php

@@ -7,6 +7,7 @@
 
 use Magento\Customer\Model\Session;
 use Magento\Framework\Data\Form\FormKey as DataFormKey;
+use \Magento\Framework\Event\Observer;
 use Magento\PageCache\Observer\FlushFormKey;
 
 class CustomerFlushFormKey
@@ -36,13 +37,15 @@ public function __construct(Session $session, DataFormKey $dataFormKey)
     /**
      * @param FlushFormKey $subject
      * @param callable $proceed
-     * @param array $args
+     * @param Observer $observer
+     * @return void
+     *
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
-    public function aroundExecute(FlushFormKey $subject, callable $proceed, ...$args)
+    public function aroundExecute(FlushFormKey $subject, callable $proceed, Observer $observer)
     {
         $currentFormKey = $this->dataFormKey->getFormKey();
-        $proceed(...$args);
+        $proceed($observer);
         $beforeParams = $this->session->getBeforeRequestParams();
         if ($beforeParams['form_key'] == $currentFormKey) {
             $beforeParams['form_key'] = $this->dataFormKey->getFormKey();

app/code/Magento/Customer/Test/Unit/Model/Plugin/CustomerFlushFormKeyTest.php

@@ -0,0 +1,106 @@
+<?php
+/**
+ * Copyright © 2013-2017 Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Customer\Test\Unit\Model\Plugin;
+
+use Magento\Customer\Model\Plugin\CustomerFlushFormKey;
+use Magento\Customer\Model\Session;
+use Magento\Framework\App\PageCache\FormKey as CookieFormKey;
+use Magento\Framework\Data\Form\FormKey as DataFormKey;
+use Magento\Framework\Event\Observer;
+use Magento\PageCache\Observer\FlushFormKey;
+use PHPUnit_Framework_TestCase as TestCase;
+use PHPUnit_Framework_MockObject_MockObject as MockObject;
+
+class CustomerFlushFormKeyTest extends TestCase
+{
+    /**
+     * @var CookieFormKey | MockObject
+     */
+    private $cookieFormKey;
+
+    /**
+     * @var Session | MockObject
+     */
+    private $customerSession;
+
+    /**
+     * @var DataFormKey | MockObject
+     */
+    private $dataFormKey;
+
+    protected function setUp()
+    {
+
+        /** @var CookieFormKey | MockObject */
+        $this->cookieFormKey = $this->getMockBuilder(CookieFormKey::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        /** @var DataFormKey | MockObject */
+        $this->dataFormKey = $this->getMockBuilder(DataFormKey::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        /** @var Session | MockObject */
+        $this->customerSession = $this->getMockBuilder(Session::class)
+            ->disableOriginalConstructor()
+            ->setMethods(['getBeforeRequestParams', 'setBeforeRequestParams'])
+            ->getMock();
+
+    }
+
+    /**
+     * @dataProvider aroundFlushFormKeyProvider
+     * @param $beforeFormKey
+     * @param $currentFormKey
+     * @param $getFormKeyTimes
+     * @param $setBeforeParamsTimes
+     */
+    public function testAroundFlushFormKey(
+        $beforeFormKey,
+        $currentFormKey,
+        $getFormKeyTimes,
+        $setBeforeParamsTimes
+    ) {
+        $observerDto = new Observer();
+        $observer = new FlushFormKey($this->cookieFormKey, $this->dataFormKey);
+        $plugin = new CustomerFlushFormKey($this->customerSession, $this->dataFormKey);
+
+        $beforeParams['form_key'] = $beforeFormKey;
+
+        $this->dataFormKey->expects($this->exactly($getFormKeyTimes))
+            ->method('getFormKey')
+            ->willReturn($currentFormKey);
+
+        $this->customerSession->expects($this->once())
+            ->method('getBeforeRequestParams')
+            ->willReturn($beforeParams);
+
+        $this->customerSession->expects($this->exactly($setBeforeParamsTimes))
+            ->method('setBeforeRequestParams')
+            ->with($beforeParams);
+
+        $proceed = function ($observerDto) use ($observer) {
+            return $observer->execute($observerDto);
+        };
+
+        $plugin->aroundExecute($observer, $proceed, $observerDto);
+    }
+
+    /**
+     * Data provider for testAroundFlushFormKey
+     *
+     * @return array
+     */
+    public function aroundFlushFormKeyProvider()
+    {
+        return [
+            'valid form key' => ['form_key_value', 'form_key_value', 2, 1],
+            'corrupted form key' => ['form_old_key_value', 'form_key_value', 1, 0],
+            'missed form key' => [null, 'form_key_value', 1, 0]
+        ];
+    }
+}