contrib/seeds/nodes_test.txt seems to be completely outdated.
I get not one outgoing onion connection using the following bitcoin.conf in which the testnet onion nodes are added with addnode= option (assuming Tor listening on 127.0.0.1:9050):
testnet=1
onion=127.0.0.1:9050
listen=1
discover=1
onlynet=onion
# contrib/seeds/nodes_test.txt
addnode=thfsmmn2jbitcoin.onion
addnode=it2pj4f7657g3rhi.onion
addnode=nkf5e6b7pl4jfd4a.onion
addnode=4zhkir2ofl7orfom.onion
addnode=t6xj6wilh4ytvcs7.onion
addnode=i6y6ivorwakd7nw3.onion
addnode=ubqj4rsu3nqtxmtp.onion