test: Fix intermittent p2p_fingerprint issue

Summary:

A single sync_with_ping can't be used to drop a block announcement, as the block might be sent after the ping has been responded to.

Fix that by waiting for the block.

Note: The useless [0] after generatetoaddress was a leftover that should have been removed in D10804.

This is a backport of core#20466

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10893