Finally I found a way to reuse a tester list.
- Open your developer console
- Click “settings” at the bottom in the left navigation bar
- Now, again in the left navigation bar the second item from bottom reads “Manage email lists”
- If you “edit” a list in this view, you can at least change the name of the list
- This is the way to reuse existing lists
I know, this is not the same as a “delete” of a list, but if you just rename a no-longer-used list and fill it with new names it’s quite the same.
you can rename your currently not-in-use lists to start with zzz or something to have them at the bottom of the list.
Hope this helps!
Cheers