async_tracking_misattribution_bug test is sporadically but consistently failing on Firefox ESR

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So far just on Firefox ESR.

Example jobs:

• https://travis-ci.org/EFForg/privacybadger/jobs/252110537
• https://travis-ci.org/EFForg/privacybadger/jobs/252201862
• https://travis-ci.org/EFForg/privacybadger/jobs/252201853

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Blocked by #1490.
Update: All clear.

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Another failure on ESR, #1517 may not help in this case: https://travis-ci.org/EFForg/privacybadger/jobs/255684572