WP_TESTS_DIR is 

current dir is /var/www/clients/client14/web33/web/wp-content/plugins/simple-fields/tests
<br />
<b>Warning</b>:  require_once(/var/www/clients/client14/web33/web/wp-content/plugins/simple-fields/tests/../tmp/wordpress-tests/includes/functions.php): failed to open stream: No such file or directory in <b>/var/www/clients/client14/web33/web/wp-content/plugins/simple-fields/tests/bootstrap.php</b> on line <b>11</b><br />
<br />
<b>Fatal error</b>:  require_once(): Failed opening required '/var/www/clients/client14/web33/web/wp-content/plugins/simple-fields/tests/../tmp/wordpress-tests/includes/functions.php' (include_path='.:/usr/share/php') in <b>/var/www/clients/client14/web33/web/wp-content/plugins/simple-fields/tests/bootstrap.php</b> on line <b>11</b><br />