GraphQL-129. Add generate customer token test

dev/tests/api-functional/testsuite/Magento/GraphQl/Customer/GenerateCustomerTokenTest.php

@@ -0,0 +1,66 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\GraphQl\Customer;
+
+use Magento\TestFramework\TestCase\GraphQlAbstract;
+
+class GenerateCustomerTokenTest extends GraphQlAbstract
+{
+
+    /**
+     * Verify customer token with valid credentials
+     *
+     * @magentoApiDataFixture Magento/Customer/_files/customer.php
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
+     */
+    public function testGenerateCustomerValidToken()
+    {
+        $userName = '[email protected]';
+        $password = 'password';
+
+        $mutation
+            = <<<MUTATION
+mutation {
+	generateCustomerToken(
+        email: "{$userName}"
+        password: "{$password}"
+    )
+}
+MUTATION;
+
+        $response = $this->graphQlQuery($mutation);
+        $this->assertArrayHasKey('generateCustomerToken', $response);
+        $this->assertInternalType('string', $response['generateCustomerToken']);
+    }
+
+    /**
+     * Verify customer with invalid credentials
+     *
+     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
+     */
+    public function testGenerateCustomerTokenWithInvalidCredentials()
+    {
+        $userName = '[email protected]';
+        $password = 'bad-password';
+
+        $mutation
+            = <<<MUTATION
+mutation {
+	generateCustomerToken(
+        email: "{$userName}"
+        password: "{$password}"
+    )
+}
+MUTATION;
+
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('GraphQL response contains errors: ' .
+            'The account sign-in was incorrect or your account is disabled temporarily. Please wait and try again later.');
+        $this->graphQlQuery($mutation);
+    }
+}