Switch focus to a particular tab by its number. It waits tabs loading and then switch tab.

```js
I.switchToPreviousTab();
I.switchToPreviousTab(2);
```

@param {number} [num] (optional) number of tabs to switch backward, default: 1.
@param {number?} [sec] (optional) time in seconds to wait.
[!] returns a _promise_ which is synchronized internally by recorder