Specify PHPUnit_Framework_TestCase as a global class

If a namespace is provided in a test file and the example from this documentation is used, then PHPunit mysteriously fails with a "Class not found" error.

This adds the `\` character to prevent this occurring.  It should work fine even if the file is not namespaced.

See http://stackoverflow.com/questions/6065730/why-fatal-error-class-phpunit-framework-testcase-not-found-in for evidence of the chain of issues that follow on when trying to fix this.