Make sure to delete existing/to save translations after each test (bug 1076992)

https://bugzilla.mozilla.org/show_bug.cgi?id=1076992

`ESTestCase` inherits from `TransactionTestCase` and therefore we need to cleanup after ourselves, and that includes translations.

Partly stolen from @magopian's work in mozilla/olympia#366
Ah yes, I fixed the indent but forgot the extra newlines.