More client credentials test

tests/api/ClientCredentialsInvalidClientSecretCept.php

@@ -0,0 +1,17 @@
+<?php
+$I = new ApiTester($scenario);
+$I->wantTo('get an access token using the client credentials grant, invalid client secret');
+$I->sendPOST(
+    'client_credentials.php/access_token',
+    [
+        'grant_type'    => 'client_credentials',
+        'client_id'     => 'myawesomeapp',
+        'client_secret' => 'foobar'
+    ]
+);
+$I->canSeeResponseCodeIs(401);
+$I->canSeeResponseIsJson();
+$I->seeResponseContainsJson([
+    'error'   => 'invalid_client',
+    'message' => 'Client authentication failed.'
+]);