Add tests to cover what is testable in SelfUpdate command #505

Job	Run time
Code coverage (codecov)	30s
CI test (make validate)	34s
Build moodle-plugin-ci.phar	45s
Integration tests (8.2, master)	7m 6s
Integration tests (8.2, MOODLE_403_STABLE)	7m 19s
Integration tests (8.2, MOODLE_402_STABLE)	9m 14s
Integration tests (8.1, MOODLE_401_STABLE)	9m 11s
Integration tests (8.0, MOODLE_400_STABLE)	6m 59s
Integration tests (8.0, MOODLE_311_STABLE)	7m 31s
Integration tests (7.4, MOODLE_310_STABLE)	7m 26s
Integration tests (7.4, MOODLE_39_STABLE)	6m 11s
Integration tests (7.4, MOODLE_38_STABLE)	6m 23s
Integration tests (PHAR) (8.2, master)	5m 52s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	6m 50s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 45s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	6m 42s
	1h 35m 18s

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