Fix race to check chaintips in p2p_headers_sync_with_minchainwork

Summary: node3 is not guaranteed to have finished syncing headers by the time getchaintips is first called.

Test Plan:

./test/functional/test_runner.py p2p_headers_sync_with_minchainwork.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17368