If it;'s similar to the 777...
Yes, hold until complete. 'FIRE TEST IN PROGRESS' will be replaced with 'FIRE TEST PASS' if the test is successful.
The Maintenance Manual (again for the 777, assuming the 787 system architecture is similar) says...
Push and hold the fire/overheat test switch. Make sure these indications come on:
* APU fire warning light
* Forward cargo fire warning light
* Aft cargo fire warning light
* Master warning lights
* Fire aural warning
* Engine fuel control switch fire warning lights * Left and right engine fire warning lights
* FIRE TEST IN PROGRESS warning message.
Release the fire/overheat test switch when the test is complete. The test is complete when the warning message FIRE TEST PASS or FIRE TEST FAIL shows. All the test indications go away when you release the switch.