Render unit results progressively

Summary:
Some tests take longer (fixtures usually around 1 second for me) and also FB runs all tests on deploy.
I want to see all results immediately.

Test Plan:
Added `usleep(200000)` to `resultTest()`, then:

  $ arc unit
  Saw results printed one by one.

Also didn't pass `$renderer` to `ArcanistPhutilTestCase` and saw empty output.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5141