Add phpunit test for decoding #423

Job	Run time
ci / check if job needs to run	13s
ci / prepare test matrix	40s
ci / MOODLE_310_STABLE - pgsql - php 7.2 - mod_cms	3m 26s
ci / MOODLE_311_STABLE - mariadb - php 7.3 - mod_cms	2m 49s
ci / MOODLE_39_STABLE - mariadb - php 7.2 - mod_cms	3m 12s
ci / MOODLE_39_STABLE - pgsql - php 7.2 - mod_cms	3m 8s
ci / MOODLE_39_STABLE - mariadb - php 7.3 - mod_cms	3m 0s
ci / MOODLE_39_STABLE - pgsql - php 7.3 - mod_cms	3m 33s
ci / MOODLE_401_STABLE - pgsql - php 7.4 - mod_cms	3m 29s
ci / MOODLE_400_STABLE - pgsql - php 7.3 - mod_cms	3m 22s
ci / MOODLE_401_STABLE - mariadb - php 7.4 - mod_cms	3m 12s
ci / MOODLE_401_STABLE - pgsql - php 8.0 - mod_cms	3m 27s
ci / MOODLE_401_STABLE - mariadb - php 8.0 - mod_cms	3m 22s
ci / Release to the Moodle plugin directory	0s
	36m 53s

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