Fix safe_lxml. SEC-338

The imports were sorted in May, which broke the monkeypatching in
safe_lxml.  I added two tests that the XML parsers are properly patched,
but they didn't pass until I added the monkeypatching to the start of
the test runs.  Once that was done, some tests failed because they
relied on specific details of how empty elements are represented.  Those
tests are now fixed.