Stephan Hochdörfer has a quick post showing how he has PHPunit up and working on an HHVM instance. His problem was that the tests were actually executing using the "php" binary, not the HHVM one.
For quite some time we run the unit tests for our libs and tools against PHP and HHVM, at least that is what I thought up to now. As it turns out I missed a minor detail. [...] What happens now is that Phing is executed via HHVM but PHPUnit will still be executed via the PHP binary because the PHPUnit shell script will look for the php binary in the PATH configuration. Since we run HHVM side-by-side with PHP on our Jenkins build nodes I was not able to point /usr/bin/php to /usr/bin/hhvm - which would be the easiest and cleanest solution. I
He shares the workaround he created, creating a symbolic link between the hhvm and php binaries and then executing the Phing task to run the tests. This is being run via Jenkins and uses it's "WORKSPACE" as a container so the main "php" binary isn't overwritten.