Add some selfupdate integration tests #569

Job	Run time
CI test (make validate)	32s
SelfUpdate tests (PHAR) (8.2)	39s
SelfUpdate tests (PHAR) (8.1)	28s
Code coverage (codecov)	0s
SelfUpdate tests (PHAR) (8.0)	22s
SelfUpdate tests (PHAR) (7.4)	8s
Integration tests (8.2, master)	1m 40s
Build moodle-plugin-ci.phar	38s
Integration tests (8.2, MOODLE_403_STABLE)	1m 41s
Integration tests (8.2, MOODLE_402_STABLE)	1m 41s
Integration tests (8.1, MOODLE_401_STABLE)	1m 39s
Integration tests (8.0, MOODLE_400_STABLE)	1m 40s
Integration tests (8.0, MOODLE_311_STABLE)	2m 24s
Integration tests (7.4, MOODLE_310_STABLE)	1m 51s
Integration tests (7.4, MOODLE_39_STABLE)	1m 40s
Integration tests (7.4, MOODLE_38_STABLE)	1m 40s
Integration tests (PHAR) (8.2, master)	48s
Integration tests (PHAR) (8.1, MOODLE_403_STABLE)	51s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	44s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	59s
	22m 5s

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