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

ESTestCase inherits from TransactionTestCase and therefore we need to
cleanup after ourselves, and that includes translations.

Also, wrap tests that create translations in a TestCase class
Merge pull request #2782 from diox/fix-translations-flaky-test

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