what did you read that made you think it wouldn't test the coinbase transaction?
I ran
bitcoin-cli getblocktemplate '{"rules": ["segwit"]}'
(after some tries, this was the only syntax that worked)
A bunch of transactions showed but I noticed that there was no coinbase transaction (there were fields at the end of the output that may be used to create the coinbase, but no formatted coinbase transaction)
I searched and found this
https://stackoverflow.com/questions/42334832/has-the-bitcoin-getblocktemplate-response-json-changed-where-is-the-coinbasetxnwhere it said
bitcoin core doesn't provide a coinbasetxn in the block template; it's the pool software that does that. if you're coding mining software to connect directly to bitcoind without using a pool, you'll need to generate the coinbase transaction yourself.
So given that the "proposal" option doesn't interact with the pool software and that I want to test everything including the pool software's ability to properly create a codebase transaction, the proposal option didn't appear to do what I wanted.
I'm working toward setting up Testnet.