Duration Reporting

Duration Reporting
Structures like this are (for me, at least?) hard to read. I don't think we need to be afraid of writing lines.

How about
```python
tests = {
    '_sub': {
        'tests': {
            '_sub': {}, 
            '_stats': {
                'passed': 0,
                'failed': 0,
                'skipped': 0,
                'error': 0,
                'xpassed': 0,
                'xfailed': 0
            },
            '_duration': 0
        }
    },
    '_stats': {
        'passed': 0,
        'failed': 0,
        'skipped': 0,
        'error': 0,
        'xpassed': 0,
        'xfailed': 0
    },
    '_duration': 0
}
```
?

Or, more DRY:
```python
from copy import deepcopy
_tests_tpl = {
    '_sub': {},
    '_stats': {
        'passed': 0,
        'failed': 0,
        'skipped': 0,
        'error': 0,
        'xpassed': 0,
        'xfailed': 0
    },
    '_duration': 0
}
tests = deepcopy(_tests_tpl)
tests['_sub']['tests'] = deepcopy(_tests_tpl)
```
?