Is it possible to package PHPUnit tests as a PHAR archive, and run them using phpunit?

I've created a .phar with the follow script:

$cPhar = new Phar('mytests-archive.phar', 0);
$cPhar->addFile('mytest.php');

$stub = <<<ENDSTUB
#! /usr/bin/php
<?php
Phar::mapPhar('mytest-archive.phar');
require 'phar://mytests-archive.phar/mytest.php';
__HALT_COMPILER();
ENDSTUB;

$cPhar->setStub($stub);
$cPhar->compressFiles(Phar::GZ);
$cPhar->stopBuffering();

But when I then try running the resulting archive as follows:

phpunit mytests-archive.phar

I get the error message:

#! /usr/bin/php
PHPUnit 3.3.17 by Sebastian Bergmann.

Class MyTestClass could not be found in /path/to/mytests-archive.phar

Does PHPUnit not support PHAR files, or am I missing a step in my build script? (This is my first attempt at using PHAR)