Add some selfupdate integration tests #567

Job	Run time
SelfUpdate tests (PHAR) (8.2)	22s
Code coverage (codecov)	0s
CI test (make validate)	31s
SelfUpdate tests (PHAR) (8.1)	19s
SelfUpdate tests (PHAR) (8.0)	20s
SelfUpdate tests (PHAR) (7.4)	13s
Build moodle-plugin-ci.phar	31s
Integration tests (8.2, master)	5m 19s
Integration tests (8.2, MOODLE_403_STABLE)	5m 13s
Integration tests (8.2, MOODLE_402_STABLE)	5m 16s
Integration tests (8.1, MOODLE_401_STABLE)	6m 4s
Integration tests (8.0, MOODLE_400_STABLE)	5m 46s
Integration tests (8.0, MOODLE_311_STABLE)	5m 23s
Integration tests (7.4, MOODLE_310_STABLE)	5m 7s
Integration tests (7.4, MOODLE_39_STABLE)	5m 10s
Integration tests (7.4, MOODLE_38_STABLE)	5m 22s
Integration tests (PHAR) (8.2, master)	5m 23s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	4m 47s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	5m 35s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 17s
	1h 11m 58s

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