assertTrue fails to output message

log_test.php

<?php
require_once(dirname(__FILE__) . '/simpletest/autorun.php');
require_once('../classes/log.php');

class TestOfLogging extends UnitTestCase {
	function __construct() {
		parent::__construct('Log test');
	}

	function testCreatingNewFile() {
		unlink(dirname(__FILE__) . '/../temp/test.log');
		$log = new Log(dirname(__FILE__) . '/../temp/test.log');
		$this->assertFalse(file_exists(dirname(__FILE__) . '/../temp/test.log'), 'Created before message');
		$log->message('Should write this to a file');
		$this->assertTrue(file_exists(dirname(__FILE__) . '/../temp/test.log'), 'File created');
	}

}
?>

output when assertFalse runs

Commenting out unlink .... in log_test.php causes assertFalse to output message as expected

log_test.php
1) Created before message at [/var/www/html/tutorial/tests/log_test.php line 13]
	in testCreatingNewFile
	in Log test
FAILURES!!!
Test cases run: 1/1, Passes: 1, Failures: 1, Exceptions: 0

output when assertTrue runs

$this->assertTrue(file_exists(dirname(__FILE__) . '/../temp/test.log'), 'File created'); does not output File created
log_test.php
OK
Test cases run: 1/1, Passes: 2, Failures: 0, Exceptions: 0