Fix check for function wddx_deserialize in tests for hhvm

The @requires just works for other tests, not for php functions.

This fixed a failure for the hhvm test server:
1) ApiFormatWddxTest::testValidSyntax
Failed asserting that null is of type "array".

Change-Id: Iadd8238ce62d4803cdf56508886ad016f8ac8f1d