Can't run tests on cygwin

[yevgenybezman]

The common module tries to rmtree the temporary directory created for each test.
Since the library file is still open, and Windows doesn't allow deleting an open file, this fails.
Adding True as the second argument (ignore errors) fixes the problem (tests pass, directory not deleted). A more thorough fix is needed.

[sampsyo]

That's unfortunate. I'm guessing this is a problem on vanilla Windows too, not just Cygwin.

I suppose we need to be more careful about closing the library DB on cleanup.

[yevgenybezman]

Still doesn't pass, but much better. I'll fix what's left.

[sampsyo]

See #670 for some notes on general Windows stuff I've been fixing (and been stymied by).