Actually squelch Harbormaster "test passed" mail

Summary:
Ref T1049. When Harbormaster tests pass, don't bother sending an email about it.

(I tried to implement this earlier but didn't test it entirely properly, and we needed a little more code.)

Test Plan: Used `bin/harbormaster build` to build some junk, got no email about passes.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8813