qa: Fix inv race in example_test
qa: Fix inv race in example_test

Summary: Backport of Core PR11025

Test Plan: test_runner.py example_test

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1975

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