Fixed issue with including including PHPUnit's Autoload script.

This pull request makes two small changes to CakeTestSuiteDispatcher::loadTestFramework().

First, it resolves any issues with vendor paths ending with a DS character by stripping the character from the end of the vendor path and only adding it where necessary.

Second, it now returns TRUE or FALSE instead of the return value from PHPUnit/Autoload.php.  PHPUnit/Autoload.php returns NULL when included successfully, which causes the result to be treated as a false value in CakeTestSuiteDispatcher::_checkPHPUnit().