Fix #30. Drop references to TestCase instances.
Fix #30. Drop references to TestCase instances.

Allow the garbage collector to free TestCase instances after they
run.

[#30]: https://github.com/danielfm/unittest-xml-reporting/issues/30

haiyangd/unittest-xml-reporting,tkanemoto/unittest-xml-reporting,NightBlues/unittest-xml-reporting,arcivanov/unittest-xml-reporting