[qa] Fix race condition in example_test.py

Summary:
PR description:

There's a race between sending a getdata for a bunch of blocks with the node receiving those blocks from a peer, which could cause test failure. Fix this.

Motivated by observing this race caused an appveyor test failure on another PR

Backport of Core PR16459

Test Plan: ninja && test/functional/test_runner.py example_test

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7886