Wait for connection to open in bip9-softforks.py

Summary:
bip9-sofforks.py stop-starts the bitcoind node twice during the test
run, but it doesn't wait for the connection from mininode to open before
continuing with the test. This leads to race conditions where the test
can fail getblocktemplate() because it has no p2p connections.

PR:https://github.com/bitcoin/bitcoin/commit/1f3d78b4e099f22b03ffeec9e91c02de81430702

Solves: T144

Test Plan: ./test/functional/test_runner.py bip9-softforks

Reviewers: schancel, O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D972