Yup, Testdisk would be run on whatever data you managed to recover with ddrescue.
You can run Testdisk on a hard drive directly, but I'd strongly recommend against that in your current case b/c you'd end up accidentally creating more bad sectors in the process. (the more you use the bad drive the more bad sectors are going to start popping up).. you prob already know that
(on the other hand, if you're ever dealing with a drive that had a bad system crash that wiped out the partition tables Testdisk would be able to fix that for you)
One other thing I forgot to mention, if the drive ends up being so scrambled that, even after Testdisk, you're not seeing anything when you plug it into your Mac, then another option is to try out Photorec
on the recovered data. It'll basically try to find any readable files inside the recovered data & let you copy them somewhere else, similar to Recuva & other apps in that category. You usually won't get to that point, but mentioned it just in case.
For what it's worth, last time I did this I found this site useful:
..they'll talk about using ddrescue to save the data into an image file, but if you're short on hard drives & USB adapters you can save the data directly to another drive as long as the other drive is big enough.