If layoutsDir is configured, allow specifying layouts without the parent directory

I found it quite strange that when specifying a layout in locals you need to put the parent directory also:

```
 res.render('veggies', {
    layout: 'layout/veggie'
  });
```

Makes more sense that if a layoutsDir is specified, you only need to specify the layout name:

```
 res.render('veggies', {
    layout: 'veggie'
  });
```

I also fixed up the tests by setting up the app in a beforeEach block, i noticed that as it was now, the default test app and the app-layoutsDir test app were not properly isolated, so for example it happened that while running the default app the layoutsDir was configured, when it should not have been, which could lead to false positives.