无法在测试文件夹外运行PHPUnit

所以我有一个CodeIgniter项目，我试图使用PHPUnit与它。我的目录看起来像

application
node_modules
Gruntfile.js
scripts
vendor
tests
    PostTest.php
    phpunit.phar
    bootstrap.php
    phpunit.xml
composer.json
package.json

当我在测试文件夹内时，我可以执行"phpunit ."或"php phpunit. "在PostTest.php中运行测试，这很好，但是当我在根文件夹中，并尝试做"phpunit测试"时，我得到错误:

ERROR: Not Found
    The controller/method pair you requested was not found.

PostTest.php

<?php
class PostTest extends PHPUnit_Framework_TestCase {
      private $CI;
      public function setUp() {
          $this->CI = &get_instance();
      }
      public function testNotes() {
        $this->CI->load->model('class');
        $students = $this->CI->class->getStudents('11');
        $this->assertEquals(3, count($students->result_array()));
      }
}

phpunit.xml

<?xml version="1.0" encoding="UTF-8"?>
    <phpunit bootstrap="bootstrap.php"
             colors="true"
             convertErrorsToExceptions="true"
             convertNoticesToExceptions="true"
             convertWarningsToExceptions="true"
             processIsolation="false"
             stopOnFailure="false"
             syntaxCheck="false"
             verbose="true">
    </phpunit>

有人知道为什么我得到这个错误吗?