Currently, Firebase Test Lab supports Robo Test (beta, March 2023) on iOS.
Bitrise has authored a Step: Virtual Device Testing for iOS that only supports XCTest.
It would be great to have a Step written by Bitrise that handles the archiving and uploading of an *.ipa to Firebase Test Lab to execute Robo Test. Supporting Robo Script would be a nice addition too.
I suppose a current work-around is to manually use Flank https://bitrise.io/integrations/steps/flank and presumably specify a Robo test type rather than instrumentation.