R1:78bd6c1ce49f

Ensure that `PHUIInvisibleCharacterTestCase` is compatible with PHP 5.3

Summary: This diff fixes the `PHUIInvisibleCharacterTestCase` unittest which is only broken when running in PHP 5.3.

Test Plan:
I wasn't able to run `arc unit` successfully for some reason.
```
arc diff master
Linting...
LINT OKAY No lint problems.
Running unit tests...
PHP Fatal error: Class PhabricatorUser contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (PhutilPerson::getSex)…