Add a finalizer to automatically delete the environment, element and category created by some tests

These three object types were being left behind and cluttering up the database. I have run a cleanup script to get rid of all of the old ones, and this pull request introduces automatic cleanup of them by the tests.

I ran it a couple of times [1][2] and verified that nothing is being left behind.

[1] http://qa-selenium.mv.mozilla.com:8080/job/moztrap.dev.saucelabs.teardown_test/20/
[2] http://qa-selenium.mv.mozilla.com:8080/job/moztrap.dev.saucelabs.teardown_test/21/