Add assertions to every test in libphutil/

Summary: Ref T4570. Add an assertion to all tests which pass by not exploding, so we can fail tests which make no assertions.

Test Plan: `arc unit --everything` with the test engine set to fail on no assertions.

Reviewers: leebyron, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4570

Differential Revision: https://secure.phabricator.com/D8435