Allow selenium image configuration #586

Job	Run time
CI test (make validate)	1m 3s
Code coverage (codecov)	59s
SelfUpdate tests (PHAR) (8.2)	32s
SelfUpdate tests (PHAR) (8.1)	16s
SelfUpdate tests (PHAR) (8.0)	27s
SelfUpdate tests (PHAR) (7.4)	8s
Build moodle-plugin-ci.phar	30s
Integration tests (8.3, main)	6m 31s
Integration tests (8.2, MOODLE_403_STABLE)	6m 20s
Integration tests (8.2, MOODLE_402_STABLE)	6m 0s
Integration tests (8.1, MOODLE_401_STABLE)	6m 47s
Integration tests (8.0, MOODLE_400_STABLE)	6m 24s
Integration tests (8.0, MOODLE_311_STABLE)	5m 45s
Integration tests (7.4, MOODLE_39_STABLE)	5m 59s
Integration tests (PHAR) (8.3, main)	6m 1s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 43s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 36s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	7m 15s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 49s
	1h 18m 5s

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