Use $_SERVER to compare environment variables in tests

Fixes reactphp/react#302

@psychoticmeow: Can you confirm if this resolves the test failure you were seeing on OSX?

I believe this might have been due to the [variables-order](http://us.php.net/manual/en/ini.core.php#ini.variables-order) INI option, and `$_ENV` often being empty by default. Using `$_SERVER` seems more reliable.