Bug 1076169 - [Camera] fix camera tests with a newer mocha (again) r=wil...

...sonpage
Not sure I understand the comment :)

Here:
* I replace `this.clock = sinon.useFakeTimers();` by `this.clock = this.sandbox.useFakeTimers();` (like below)
* I remove the teardown because it's made useless by the use of the sandbox

Am I missing something?