MNT Specify primary DB connection in unit test #539

Job	Run time
CI / Context	1s
CI / Generate matrix	20s
CI / 8.1 prf-low mysql57 phpunit all	1m 58s
CI / Check governance	0s
CI / 8.2 mariadb phpunit all	2m 11s
CI / 8.3 mysql80 phpunit all	2m 45s
CI / 8.1 mysql57 phplinting	1m 21s
CI / 8.1 mysql57 endtoend root	1m 9s
CI / 8.3 mysql80 endtoend root	1m 19s
CI / 8.1 mysql57 js	1m 55s
CI / 8.1 mysql57 doclinting	2m 29s
CI / Dispatch tag patch release	0s
	15m 28s

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