You can assign nodes to environments when bootstrapping, but I already have a bunch of production nodes sitting in _default.
I could use knife to just assign them to other environments or I was thinking of having a role for each environment that has a recipe which runs chef-client::config and sets the environment attribute, then I could just assign nodes to the right environment role. The problem with using a role I think though would be the extra hoops I'd have to jump through to hook things up in test kitchen.
How do chef people usually solve this problem?
The right place to start is knife exec with a transform script.
This should probably get you started:
knife exec -E 'nodes.transform("roles:foo") {|n| n.chef_environment = "foo" }'
The argument there is a search query (you can also use "*:*" to grab all the nodes) and then do something with each node object in the block. If the return value of the block is true-y it gets saved, if false-y it doesn't.