set function name on new test function

at https://github.com/notion/dado/blob/master/dado/dado.py#L63 - the `build_test` function both builds the function, and builds the function's new name, and then returns both as a tuple.

It should set the function's name on the function and just return the function.  The caller can then set that function in the module like `setattr(module, func.__name__, func)`
