[Slim] Generation of UnitTest stubs (#566)

* [Slim] Add PHPUnit4 to Composer

* [Slim] Configure PHPUnit

* [Slim] Add Model and Api test templates

* [Slim] Refresh Slim samples

samples/server/petstore/php-slim/lib/Model/FileSchemaTestClass.php

@@ -0,0 +1,18 @@
+<?php
+/**
+ * FileSchemaTestClass
+ */
+namespace OpenAPIServer\Model;
+
+/**
+ * FileSchemaTestClass
+ */
+class FileSchemaTestClass {
+
+    /** @var \OpenAPIServer\Model\File $file */
+    private $file;
+
+    /** @var \OpenAPIServer\Model\File[] $files */
+    private $files;
+
+}