bugfix: don't use sys.stdout as a default arg value (#16541)

After migrating to `travis-ci.com`, we saw I/O issues in our tests --
tests that relied on `capfd` and `capsys` were failing. We've also seen
this in GitHub actions, and it's kept us from switching to them so far.

Turns out that the issue is that using streams like `sys.stdout` as
default arguments doesn't play well with `pytest` and output redirection,
as `pytest` changes the values of `sys.stdout` and `sys.stderr`. if these
values are evaluated before output redirection (as they are when used as
default arg values), output won't be captured properly later.

- [x] replace all stream default arg values with `None`, and only assign stream
      values inside functions.
- [x] fix tests we didn't notice were relying on this erroneous behavior

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