Close mysql connection in TestXMySQL to prevent tests freezing
The unit tests freeze on my machine without this change because you can't drop a table until other connections that are using that table disconnect.

Thanks! 
Can confirm this fixes it (have to admit I almost never ran those mysql tests locally ..)
 Will just wait on travis to merge when green.
 Do you have the same issue locally before this change?
 yes, it specifically hangs on the `test_execute_closed_connection` test for me. But strange that it runs on travis OK. I almost never ran those old tests (`TestXMySQL`) locally, but relied on travis for these.

(BTW, we should clean this up once, remove that entire class, but check if there is still something tested there that is not tested in the other tests)
