Amusingly something very similar is already possible.
I made new player nation which I just called "Mars Test Center", gave them a Flag that had the nation flag i the top corner, gave them 0 pop, no res or SY. So a empty non functional player if you will. Set the relationship to fixed sum whatever you like(dont think it matters) and make them Allied.
As ships are about to be Mothballed I give them over to this faction and they just hang around.
Usually I will load them with older AMM (sometimes some newer as well) and Training Missiles (Same size and powertrain as my best missiles but with WH to 0 and adjusted with extra fuel. )
AS for testing, Turn each other to hostile and commence exercise.
You will now be able to test your own systems up against your own missiles, range, intercept %, if Test Center missiles reach through your defences they will indicate hit with 0 damage
safe way to find good AAM doctrines and wpns.
As for non missile units, I either have them attack old cargo ships which are decommissioned,or I plan to make a armoured Ortbital which they can shot at.
Since its hostile they gain combat experience, I found this to be quite useful, In testing my units and stances, detection ranges and so on.
If you salvage enemy components, you can custom build a ship with they components and then test your ships up against their sensor and wpn systems to see if you will get detected.
And having a orbital in each fleet as target, you can even see approx how much damage would be dealt to each side in a gun fight.
Or you could have Livex