Fix getbestblockhash() called before the tip reaches the expected block
This fix a bug raised on CI (Teamcity #3004).
Analyzing the log shown that delay_node.getbestblockhash() was called
before its tip has been updated to alt_node_new_tip, which occured 3ms
later. Waiting for the tip to settle should avoid the issue.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2221