Update bootstrap.php

Hello,
When running tests with processIsolation="true" on phpunit.xml, the bootstrap file is executed for each file. So to run these tests, there is an error: PHPUnit_Framework_Exception : Notice: Constant CMF_TEST_ROOT_DIR already defined in vendor\symfony-cmf\testing\bootstrap\bootstrap.php on line 32

This update resolve the error.

Regards,
Merge pull request #31 from ApiciusPoulet/patch-1

Update bootstrap.php to launch tests in isolation