Bug 1020757 - [FDN] Enable/Disable FDN asks to enter PUK if the enable/disable FDN failed [r=francisco]

Sergi, you can check this : https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/test/unit/simcard_manager_test.js#L142

Basically, you can use `this.sinon.*` in tests, so that you don't have to write redundant `xxx.restore()` in teardown block and this is really handy.

For more reference you can check : https://github.com/mozilla-b2g/gaia/blob/master/dev_apps/test-agent/common/test/sinon_helper.js