Test discovery does not account for .hh file extensions

Resolves #1389

In essence, this is another incarnation of https://github.com/sebastianbergmann/phpunit/pull/1369#issuecomment-50702040, root cause:

> Without resetting `$newClasses = array();`, all of the validations are executed and they fail, but the original list of newly declared classes is still defined, so all of the arbitrary new classes are added to the test suite.

This PR does not address the root cause; it's only a stop-gap to support `.hh` file extensions.