Use explicit phase rather than job.phases[0] to avoid flakiness related to unordered-ness of job.phases
Use explicit phase rather than job.phases[0] to avoid flakiness related to unordered-ness of job.phases

Summary:
- job.phases is ordered by date_started, but e.g. in the case of tests
  date_started is NULL so it ends up not having a consistent ordering.
- My first flaky test!

Test Plan: Unit tests, hopefully consistently now.

Reviewers: anupc, kylec

Reviewed By: kylec

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D154756

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