Add tests to cover what is testable in SelfUpdate command #566

Job	Run time
Code coverage (codecov)	36s
CI test (make validate)	33s
SelfUpdate tests (PHAR) (8.2)	40s
SelfUpdate tests (PHAR) (8.1)	22s
SelfUpdate tests (PHAR) (8.0)	26s
SelfUpdate tests (PHAR) (7.4)	13s
Build moodle-plugin-ci.phar	25s
Integration tests (8.3, main)	6m 42s
Integration tests (8.2, MOODLE_403_STABLE)	5m 58s
Integration tests (8.2, MOODLE_402_STABLE)	5m 47s
Integration tests (8.1, MOODLE_401_STABLE)	6m 53s
Integration tests (8.0, MOODLE_400_STABLE)	6m 33s
Integration tests (8.0, MOODLE_311_STABLE)	5m 57s
Integration tests (7.4, MOODLE_39_STABLE)	5m 49s
Integration tests (PHAR) (8.3, main)	5m 45s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	5m 42s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 46s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 23s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 55s
	1h 16m 25s

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