refactor: Add void to PHPUnit test methods #16597

Job	Run time
Front-end unit tests	1m 52s
master w/ php8.3-sqlite-nocache integration tests	3m 28s
master w/ php8.3-mysql-nocache integration tests	3m 53s
master w/ php8.3-pgsql-nocache integration tests	4m 0s
master w/ php8.1-sqlite-redis integration tests	4m 26s
master w/ php8.2-mysql-redis integration tests	6m 8s
stable30 w/ php8.2-mysql-redis integration tests	5m 49s
Nextcloud master php8.1 unit tests	1m 39s
Nextcloud stable30 php8.1 unit tests	1m 33s
Nextcloud master php8.2 unit tests	3m 14s
Nextcloud stable30 php8.2 unit tests	3m 4s
Nextcloud master php8.3 unit tests	2m 48s
Nextcloud stable30 php8.3 unit tests	2m 47s
test-summary	6s
	44m 47s

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