qa: Only allow disconnecting all NodeConns
Summary:
Disconnecting the connection with index=0 makes no sense when there are more than one connections, as the list "rotates around" and populates index 0 after del. Just disconnect all NodeConns in any case.
Backport of core PR11641
Test Plan:
./test/functional/test_runner.py --extended
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2356