Make fake_timers_test stable

This pull request improves stability of `fake_timers_test.js`.

* Explicitly restoring `this.global`'s attributes in `tearDown` made the test stable
* `hasOwnProperty` does not behave as expected when executed in Safari, and then skip one test in that case

The reason why `fake_timers_test.js` was so unstable is not clear yet...