[PHP] Cleanup test (#7673)

* Regenerate PetApiTest, OrderTest

The same test code as deleted is in the 'tests' directory.

* Move test code 'test' -> 'tests', and Regenerate test/Model/PetTest.php

* Regenerate 'test' directory

rm -rf samples/client/petstore/php/SwaggerClient-php/test && bin/php-petstore.sh

samples/client/petstore/php/SwaggerClient-php/tests/PetTest.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Swagger\Client;
+
+use Swagger\Client\Model\Pet;
+
+class PetTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * test empty object serialization
+     */
+    public function testEmptyPetSerialization()
+    {
+        $new_pet = new Pet;
+        // the empty object should be serialised to {}
+        $this->assertSame("{}", "$new_pet");
+    }
+}