Grunt: fix path to WP unit tests when WP_TESTS_DIR isn't set

This should fix the fake passes that we’re seeing on Travis-CI.

This change is needed as result of the trunk re-organisation caused by
the adoption of Grunt. See #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8376 cdf35c40-ae34-48e0-9cc9-0c9da1808c22