tests/bootstrap: don't re-declare PHPUNIT_RUN

This is already set by Nextcloud's bootstrap (and from what I've seen we
require that anyways), so no need to do this on our own.

This also resolves the following warnings from `make test`:

    Warning: Constant PHPUNIT_RUN already defined in /home/ma27/Projects/nextcloud/tests/bootstrap.php on line 7
    PHP Warning:  Cannot modify header information - headers already sent by (output started at /home/ma27/Projects/nextcloud/tests/bootstrap.php:7) in /home/ma27/Projects/nextcloud/lib/private/legacy/OC_Response.php on line 71

Signed-off-by: Maximilian Bosch <[email protected]>

tests/bootstrap.php

@@ -8,7 +8,6 @@
  * @author Julien Veyssier <[email protected]>
  * @copyright Julien Veyssier 2019
  */
-define('PHPUNIT_RUN', 1);
 
 require_once __DIR__.'/../../../tests/bootstrap.php';
 require_once __DIR__.'/../../../lib/base.php';