Update api_key_test.py for flaky test

[Sita04]

Fixes #8787

[loferris]

LGTM

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[kweinmeister]

@Sita04 it's hard to tell with all the build failures, but there is a Lint issue blocking this from getting merged:

./api_key_test.py:19:1: I100 Import statements are in the wrong order. 'from _pytest.capture import CaptureFixture' should be before 'import backoff'

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