Fix proxy_test logging crash when no IPv6 available
The test tried to use the self.log member in __init__()
but it is only set up in main() in BitcoinTestFramework.
This moves the calls into setup_nodes() which is called
by main, and should log correctly if IPv6 is not available.
check that the test no longer fails on Travis
Reviewers: #bitcoin_abc, deadalnix, CCulianu
Reviewed By: #bitcoin_abc, deadalnix, CCulianu
Differential Revision: https://reviews.bitcoinabc.org/D504