FIX Ensure DB is active before processing

[emteknetnz]

When using the TestSession with behat on a project while silverstripe-fulltextsearch is installed, the following chain of events happens

• Temporary database used by TestSession is destroyed
• PHP shuts down
• The following is run, which was defined in FullTextSearch - SearchUpdater.php
• register_shutdown_function(array(SearchUpdater::class, "flush_dirty_indexes"));
• flush_dirty_indexes will eventually call SearchUpdateProcessor::process() - which will do a bunch of DB queries
• however there's no database, which triggers No database selected errors