Fix bug where stacked decorators don't update

[wrongu]

Description of the bug: The partial_credit and leaderboard decorators are class-based and use a class-based decorator pattern with @functools.wraps around an inner decorator method, along with a local callback function that is passed to the tests (i.e. set_score and set_leaderboard_value). The bug is that set_score and set_leaderboard_value update attributes of the wrong function when they are stacked. This means that only the outermost decorator with a callback function works. Example of the bug:

@partial_credit(1)
@leaderboard('a')
def test_a(set_score=None, set_leaderboard_value=None):
    set_score(1)  # works
    set_leaderboard_value(1) # fails silently, updating the __leaderboard_value__ of test_a but not of the outermost wrapper.

or, with the decorators in the other order:

@leaderboard('a')
@partial_credit(1)
def test_a(set_score=None, set_leaderboard_value=None):
    set_score(1)  # fails silently
    set_leaderboard_value(1) # works

Existing workaround: One can in principle avoid these issues by having separate partial_credit and leaderboard tests. However, this is not future-proof for adding new decorators. In fact, I discovered this bug because I added my own custom decorator for timeout, and I found that @timeout followed by @partial_credit failed while the reverse order behaved properly.

Fix: This PR fixes the issue by essentially emulating the functools.wraps behavior after the function is called (using update_wrapper), which ensures that the new values set by set_score and set_leaderboard_value are copied to the outer wrappers and thus visible to the test runner. I opted to use a context manager for this because it makes it easy to drop in a 1-line fix inside the decorators.