Bug 911055 - [zffos1.1][P3][Dailer]DuT doesn allow to dial a <<pause>>

This var is unnecessary. It can be written like this:
```js
this.sinon.spy(MockMozTelephony, 'startTone');
```
... later...
```js
sinon.assert.notCalled(MockMozTelephony.startTone);
```