Fix an issue where method prototypes have different signatures

Summary: PHP whines about this:

> [2013-09-23 08:09:22] ERROR 2048: Declaration of CSharpToolsTestEngine::loadEnvironment() should be compatible with XUnitTestEngine::loadEnvironment($config_item = 'unit.xunit...') at [/INSECURE/devtools/arcanist/src/unit/engine/CSharpToolsTestEngine.php:13]

Auditors: jamesr, btrahan