Add tests to cover what is testable in SelfUpdate command #558

Job	Run time
CI test (make validate)	35s
SelfUpdate tests (PHAR) (8.2)	33s
Code coverage (codecov)	29s
SelfUpdate tests (PHAR) (8.1)	17s
SelfUpdate tests (PHAR) (8.0)	22s
SelfUpdate tests (PHAR) (7.4)	14s
Build moodle-plugin-ci.phar	31s
Integration tests (8.2, main)	6m 12s
Integration tests (8.2, MOODLE_403_STABLE)	6m 37s
Integration tests (8.2, MOODLE_402_STABLE)	6m 17s
Integration tests (8.1, MOODLE_401_STABLE)	6m 53s
Integration tests (8.0, MOODLE_400_STABLE)	6m 27s
Integration tests (8.0, MOODLE_311_STABLE)	6m 10s
Integration tests (7.4, MOODLE_39_STABLE)	5m 47s
Integration tests (PHAR) (8.2, main)	5m 35s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	5m 38s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 46s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 39s
	1h 11m 2s

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