Function test_instance 

pub fn test_instance(tt: Arc<TurboTasks<TurboTasksBackend>>) -> TestInstance

Wrap a concrete Arc<TurboTasks<TurboTasksBackend>> into a TestInstance. Called from the register! macro — the .trs closure returns a concrete TurboTasks, and this function erases the type while retaining eviction access via a capturing closure.