On the HHVM blog today there's a new post shows how far along they are with parity with the PHP language based on the tests from a sampling of several large PHP-based projects.

HHVM has a large suite of unit tests that must pass in several build configurations before a commit reaches master. Unfortunately, this test suite passing doesn’t tell you if HHVM can be used for anything useful – so we periodically run the test suites for popular, open source frameworks. [...] The frameworks test page is now public, as is the JSON data backing it (which you’re welcome to use).

They look briefly at what exactly is tested (latest stable version, with exceptions) and how it all works. The tests are run once an hour and are based on a completely clean build of HHVM in "csv" mode. The results of the tests are automatically pushed into the MySQL+Memcached system reporting system, accessible via the JSON API.