Daemonize bitcoind in test-seeds.sh rather than run in a background process

Summary:
test-seeds broke recently with a change to outputting debug info to the terminal
rather than a log file. This patches fixes that behavior by running bitcoind with -daemon
rather than backgrounding the process.

Test Plan: ./test-seeds.sh --testnet < nodes_test.txt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4724