Git commits in Moodle 3.3.3 by Mark Johnson

commit 8edb87a37f486710a36684aecf4c8c27c89741b4
Author: Mark Johnson <mark.johnson@open.ac.uk>
Date:   Thu, 26 Oct 2017 21:59:35 +0800
Tag:    v3.3.3

MDL-60600 core_output: Clear static icon system cache between unit tests