As reported by Raj in MDL-50565:

There are 2 notices in phpunit with current patch which are not being handled by "error Control Operators" in php7 (@)

The notices are being generated by:

1. blocks/rss_client/tests/cron_test.php
2. lib/tests/rsslib_test.php

Masking E_USER_NOTICE in test case fix the problem.

- @$block->cron();

+ error_reporting($CFG->debug & ~E_USER_NOTICE);

+ $block->cron();

+ error_reporting($CFG->debug);