Fail Arcanist tests when they make zero assertions

Summary:
Fixes T4570. When a test case doesn't make any assertions, fail it:

  - A tiny fraction of tests pass by not throwing. These tests can easily make a trivial assertion. It took about 5 minutes to fix them all (D8435, D8436).
  - In other cases, no assertions means a test construction problem, as with T4570. In these cases, failing loudly catches a severe error.
  - Fixes the no-assertion test cases in `arcanist/`
  - Makes the PHP 5.4 test pass for the moment, see discussion in T4334.

Test Plan: Ran `arc unit --everything`.

Reviewers: leebyron, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4570

Differential Revision: https://secure.phabricator.com/D8437