Better nosetest build folder naming

Windows failing to build genmsg which uses 

```
catkin_add_nosetests(${PROJECT_SOURCE_DIR})
```

and it falls all over the `C:\` prefix to the path. This led to finding this bit of code:

* Was it actually intending to strip the source directory and not the binary directory? I can't see the reason you'd want to strip the binary directory and stripping the source directory makes it more convenient for introspection.
* This regex replace code is quite a bit simpler for removing prefixes
* Provides a name when ends up with the empty string (could maybe choose a better name?)
* Also replaces colons with dots to avoid problem windows path characters.