Junit xml reporter grouping by classname

Junit reporter will generate unique testcase "classname" attribute from original filename.

```
<testcase classname="Cedar" name="Some test" time="0.002539"/>
<testcase classname="Cedar" name="Yet another test" time="0.090917"/>
<testcase classname="Cedar" name="Third one" time="0.090917"/>
```
became
```
<testcase classname="MySpec" name="Some test" time="0.002539"/>
<testcase classname="MyAnotherSpec" name="Yet another test" time="0.090917"/>
<testcase classname="MyAnotherSpec" name="Third one" time="0.090917"/>
```

When it's not possible for some reason to resolve filename from Spec, it will fallback to default "Cedar" classname

Related to #171