[Dashboard] Add TasksByGuid helper to add_histograms_test
[Dashboard] Add TasksByGuid helper to add_histograms_test

Many tests for add_histograms will want to make assertions about the tasks in
the task queue. This CL extracts the pattern used to do set this up into a
helper method called TasksByGuid.

BUG=catapult:#3499

Review-Url: https://codereview.chromium.org/2961153002

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