add DB tests for credentials manager

these are actually expected to FAIL, because NULL as a userid is not
allowed in the schema, but documented to be used on the source

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

tests/lib/Security/CredentialsManagerTest.php

@@ -27,6 +27,9 @@ use \OCP\Security\ICrypto;
 use \OCP\IDBConnection;
 use \OC\Security\CredentialsManager;
 
+/**
+ * @group DB
+ */
 class CredentialsManagerTest extends \Test\TestCase {
 
 	/** @var ICrypto */