chore(log): add `log.empty()` method to the testing logger

`log.empty()` is the same as `log.reset()`, except thati `empty()`  also returns the current array with messages

instead of:

```
// do work
expect(log).toEqual(['bar']);
log.reset();
```

do:

```
// do work
expect(log.empty()).toEqual(['bar']);
```