[PHP] Cleanup EnumTest (#7282)

* Move test codes in "test" to "tests"

- EnumClassTest
- EnumTestTest

* Regenerate test/Model/Enum(Class|Test)Test.php

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

@@ -0,0 +1,15 @@
+<?php
+
+namespace Swagger\Client;
+
+use Swagger\Client\Model\EnumClass;
+
+class EnumClassTest extends \PHPUnit_Framework_TestCase
+{
+    public function testPossibleValues()
+    {
+        $this->assertSame(EnumClass::ABC, '_abc');
+        $this->assertSame(EnumClass::EFG, '-efg');
+        $this->assertSame(EnumClass::XYZ, '(xyz)');
+    }
+}