bugfix: use test client in unit tests #777

Job	Run time
ci / check if job needs to run	4s
ci / prepare test matrix	10s
ci / MOODLE_33_STABLE - mariadb - php 7.1 - tool_objectfs	2m 8s
ci / MOODLE_34_STABLE - pgsql - php 7.1 - tool_objectfs	1m 43s
ci / MOODLE_35_STABLE - mariadb - php 7.1 - tool_objectfs	1m 45s
ci / MOODLE_35_STABLE - mariadb - php 7.2 - tool_objectfs	1m 47s
ci / MOODLE_35_STABLE - pgsql - php 7.1 - tool_objectfs	1m 50s
ci / MOODLE_37_STABLE - pgsql - php 7.2 - tool_objectfs	2m 16s
ci / MOODLE_35_STABLE - pgsql - php 7.2 - tool_objectfs	2m 1s
ci / MOODLE_36_STABLE - mariadb - php 7.1 - tool_objectfs	2m 2s
ci / MOODLE_38_STABLE - mariadb - php 7.3 - tool_objectfs	2m 24s
ci / MOODLE_39_STABLE - mariadb - php 7.2 - tool_objectfs	2m 25s
ci / MOODLE_39_STABLE - pgsql - php 7.2 - tool_objectfs	2m 32s
ci / MOODLE_39_STABLE - mariadb - php 7.3 - tool_objectfs	2m 23s
ci / MOODLE_39_STABLE - pgsql - php 7.3 - tool_objectfs	2m 33s
ci / Release to the Moodle plugin directory	0s
	28m 3s

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