Make the WP Mock TestCase class abstract

This will prevent PHPUnit from thinking it should have tests associated with it.
Merge pull request #14 from johnpbloch/abstract-testcase

Make the WP Mock TestCase class abstract
Workaround for HHVM not supporting case-insensitive constants
Merge pull request #16 from goblindegook/fix-hhvm-ci-constants

Workaround for HHVM not supporting case-insensitive constants