Grounding the transistor doesn't actually test the transistor itself, though in this case it's telling you the relay is good and the flippers are ok otherwise. You can use the diode test on your dmm to check the transistor, compare readings with other known good ones nearby. If transistor is good, I'd suspect the chip that drives the transistor.

Yep, looks like the chip driving Q15 is at U4. 2136913.png

I'd check the transistor at Q15 and the diode at CR15 as well. If they both check good, I'd suspect U4. You can also ground the metal tab on Q15 to see if the flipper relay activates. If the relay clicks/activates, you should be able to try the flippers while it's activated.

Note that grounding Q15 doesn't test the transistor itself, you'll still need to check the transistor using the diode test on your dmm.