Use nonlocal rather than box[0] in tests

[wchargin]

Summary:
To set a value in an outer scope from within a nested function, we’ve
historically used the pattern of “boxing” the value in a length-1 list:

count_box = [0]

def foo():
    count_box[0] += 1
    ...

foo()
assert count_box[0] == 1

Now that we use Python 3 everywhere, we can avoid the box and just use
the nonlocal keyword instead:

count = 0

def foo():
    nonlocal count
    count += 1
    ...

foo()
assert count == 1

This patch fixes up uses of this pattern, all of which are in tests.

Part of #4488.

Test Plan:
Running git grep -F '_box = [' no longer matches any results.

wchargin-branch: box-to-nonlocal

[nfelt]

Oh python :/ Thanks for the cleanup. If you wanna create an issue for "thank goodness we finally can stop caring about Python 2" and log cleanup like this to it you might as well get credit for it.

[wchargin]

Sure. Credit attribution aside, that will be a nice place to keep a
to-do list of “changes that I want to make in low-traffic periods to
avoid merge conflicts”, like “remove all __future__ imports”. #4488