Make assertTrue() / assertFalse() messages more descriptive

Summary: See discussion in D8460. Primarily, this prints out the failing value when a true/false assertion fails, so if it was something useful (like a function result) it's visible.

Test Plan:
Added `assertTrue("quack")`:

     FAIL  ArcanistDiffParserTestCase::testParser
  Assertion failed, expected 'true' (at ArcanistDiffParserTestCase.php:16).

  ACTUAL VALUE
  quack

Added `assertFalse("quack")`:

     FAIL  ArcanistDiffParserTestCase::testParser
  Assertion failed, expected 'false' (at ArcanistDiffParserTestCase.php:16).

  ACTUAL VALUE
  quack

Added `assertEqual("quack", "moo")`:

     FAIL  ArcanistDiffParserTestCase::testParser
  Assertion failed, expected values to be equal (at ArcanistDiffParserTestCase.php:16).
  Expected: quack
    Actual: moo

Reviewers: joshuaspence

Reviewed By: joshuaspence

CC: aran

Differential Revision: https://secure.phabricator.com/D8465