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

I prefer to use `this.sinon.stub` or `this.sinon.spy` in `setup()`, in this way, you don't have to restore them in `teardown()` here : https://github.com/mozilla-b2g/gaia/pull/25219/files#diff-ba715d750690c1a7abb4567839609f35R149