wait for db flush to complete to carry on with other test suites

fixes the problem where tests would sometimes fail due to timing issues

tests/database/sets.js

@@ -229,7 +229,7 @@ describe('Set methods', function() {
 	});
 
 
-	after(function() {
-		db.flushdb();
+	after(function(done) {
+		db.flushdb(done);
 	});
 });