Not really a tool, but an easy way to check chain tension:
Yesterday I put a new chain on my N15.
I first compress the suspension so that the sprockets are in line, and then set the chain to minimum play.
When I release the load, the chain becomes much looser.
Then I tighten it again with a 5 mm rod in a link on the lower run ( any link will do) and mark the angle of the rod to the chain on a piece of cardboard.
I keep the cardboard for reference when I later have to adjust the chain.
It's pretty accurate and keeps your fingers clean.
Hard to photograph on the bike, but I hope this is clear :
Instead of measuring up-down movement, I just look at the angle.