Upgrade: Fix Behat test which tests the activity icon branding featur… #3671

Job	Run time
moodle-plugin-ci / Preflight	7s
moodle-plugin-ci / Static checks (MOODLE_501_STABLE, 8.4, pgsql)	4m 58s
moodle-plugin-ci / Runtime tests (MOODLE_501_STABLE, 8.4, pgsql)	6h 0m 15s
moodle-plugin-ci / Runtime verification	0s
	6h 5m 20s

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