Bump tested-up-to version to 3.9. (trunk)

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5340 9866e705-20ec-0310-96e7-cbb4277adcfb
Use correct PHPUnit configuration file `phpunit.xml.dist` for Grunt task `grunt phpunit`
* Props netweb. See #2542

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5342 9866e705-20ec-0310-96e7-cbb4277adcfb