improve isolation of tests that depend on logging

Summary:
Several tests use log output, but none of them reset it. This can lead to some
tests influencing others. Design resetting log data into the API that tests use.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D922988

Task ID: 2718659

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extra : rebase_source : 2abc8f84f20fcf658260bf2d32fa0f91dbf7427f
extra : amend_source : f2e482f0ade8c40530b38c7e18441823c61246d5