Skip ApiFormatWddxTest under HHVM

wddx_serialize_value() fails to escape the ampersand under HHVM. It has
been fixed upstream https://github.com/facebook/hhvm/issues/4283 but
has not been released yet.

When running under HHVM and detecting the ampersand is not escaped, skip
the test..

Bug: T75531
Change-Id: Ia58ec20b4daf78cd90da1bdf8af6cac86015c5d7