Adds missing test

Author: nuxwin

test/UserRepository/PdoDatabaseFactoryTest.php

@@ -51,6 +51,9 @@ public function getPdoInvalidConfig()
     {
         return [
             [[]],
+            [[
+                'service' => PDO::class,
+            ]],
             [[
                 'service' => PDO::class,
                 'table'   => 'test'
@@ -112,7 +115,7 @@ public function getPdoValidConfig()
     {
         return [
             [[
-                'service' => \PDO::class,
+                'service' => PDO::class,
                 'table'   => 'user',
                 'field'   => [
                     'identity' => 'username',