contrib/seeds/nodes_test.txt seems to be completely outdated.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):
 0testnet=1
 1onion=127.0.0.1:9050
 2listen=1
 3discover=1
 4onlynet=onion
 5# contrib/seeds/nodes_test.txt
 6addnode=thfsmmn2jbitcoin.onion
 7addnode=it2pj4f7657g3rhi.onion
 8addnode=nkf5e6b7pl4jfd4a.onion
 9addnode=4zhkir2ofl7orfom.onion
10addnode=t6xj6wilh4ytvcs7.onion
11addnode=i6y6ivorwakd7nw3.onion
12addnode=ubqj4rsu3nqtxmtp.onion