This is a followup of
MDL-48837, where a fix for backup/util/helper/tests/cronhelper_test.php was added, so timezone was reset as part of its tearDown() process.
Anyway, this is a detail easy to forget, so this is an issue about to:
- Add support for TZ as part of the normal reset between tests.
- Kill all the setUp() and tearDown() occurrences handling TZs (and associated properties).