Add partial_credit function to coverage,py

Something like:
```py
def partial_credit(score, output="Partial credit.\n") -> None:
    """Give partial credit for passing some of the asserts.

    Sets the `score` and `output` attributes of the test function.
    """
    test_func = get_caller()
    test_func.score = score
    test_func.output = output
```

Also modify the `half_credit()` function to use an output parameter with default value.