Fix "to spec" transition of shell's TestDescription

[happz]

TestDescription objects have normalized fields (e.g. require: List[Union[str, 'tmt.base.FmfId']]), and these objects are converts to fmf nodes by calling their to_dict() - and this method does not produce valid, plain fmf node, but let's normalized objects sneak into the dictionary.

Which is correct, to_dict() should behave like this, we need to equip the instance with to_raw() method for this purpose.