Unit Tests: tidy up `wp_blogs` table between tests.

Sites created by the `WP_UnitTest_Factory_For_Blog` sometimes are not removed when the current transaction is rolled back. This requires further investigation to understand the root cause but for now, we'll empty out the blogs table manually.

See r9280 / #5857.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9281 cdf35c40-ae34-48e0-9cc9-0c9da1808c22