Use a temporary datadir and non-default RPC port when testing seeds

Summary: This makes it far less likely to interfere with any nodes that may already be running.

Test Plan:

bitcoind  # running a node should not cause any issues post-patch
./test-seeds.sh < nodes_main.txt
./test-seeds.sh --testnet < nodes_test.txt
../teamcity/check-seeds.sh main 80
../teamcity/check-seeds.sh test 80

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5246