Name or path should be added on configuration as well
Close https://github.com/huggingface/transformers/issues/8035

Currently a configuration initialized with 
```
config = BertConfig.from_pretrained(model_name)
```
does not have the `_model_name_or_path` attribute, whereas a configuration intialized from a model with 
```py
model = BertModel.from_pretrained(model_name)
``` 
does.

This fixes the discrepancy and fixes the failing test in the process.