"test" prefix for rewrite_logs() function of WandbCallback
# 🚀 Feature request

I would like it if the function `rewrite_logs()` of WandbCallback() would be compatible with the trainers `.predict()` function. 

## Motivation

Currently the `rewrite_logs()` function looks like this: 

```python
def rewrite_logs(d):
    new_d = {}
    eval_prefix = "eval_"
    eval_prefix_len = len(eval_prefix)
    for k, v in d.items():
        if k.startswith(eval_prefix):
            new_d["eval/" + k[eval_prefix_len:]] = v
        else:
            new_d["train/" + k] = v
    return new_d

```

but `.predict()` has an argument which is called `metric_key_prefix: str = "test"` and sets the prefix to `test`, 
This leads to the kind of ugly metric name `train/test_f1` in wandb. Better would be `test/f1` 
 

## Your contribution
This should do the trick? 

```python
def rewrite_logs(d):
    new_d = {}
    eval_prefix = "eval_"
    eval_prefix_len = len(eval_prefix)
    test_prefix = "test_"
    test_prefix_len = len(test_prefix)
    for k, v in d.items():
        if k.startswith(eval_prefix):
            new_d["eval/" + k[eval_prefix_len:]] = v
        elif k.startswith(test_prefix):
            new_d["test/" + k[test_prefix_len:]] = v
        else:
            new_d["train/" + k] = v
    return new_d

```
