Allow selenium image configuration #560

Job	Run time
Code coverage (codecov)	23s
CI test (make validate)	33s
Build moodle-plugin-ci.phar	29s
Integration tests (8.2, main)	6m 11s
Integration tests (8.2, MOODLE_403_STABLE)	5m 59s
Integration tests (8.2, MOODLE_402_STABLE)	5m 42s
Integration tests (8.1, MOODLE_401_STABLE)	7m 23s
Integration tests (8.0, MOODLE_400_STABLE)	6m 45s
Integration tests (8.0, MOODLE_311_STABLE)	6m 1s
Integration tests (7.4, MOODLE_39_STABLE)	5m 51s
Integration tests (PHAR) (8.2, main)	5m 57s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	5m 31s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 29s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 38s
	1h 8m 52s

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