Consider replacing "manual" mock classes with 'unittest.mock' versions

[tseaver]

From discussion on #3500.

There are a bunch of manually-defined stub classes in place throughout the codebase. Suggested pros for replacement:

• mock.Mock confines to the test what the expectation is, using a format that contributors are going to be familiar with. Having to look up should not be necessary.
• Improved / standardized assertions (assert_called_once_with and friends).

Cons:

• Lots of churn, potentially introducing bugs.
• DRYing out frequently-used mock setups means that users still have to go look up the helper, rather than a class: the class can be more "transparent" than the imperative setup (e.g., just add a class-scope attribute, rather than adjusting both the value passed to the mock and its scope).

[lukesneeringer]

I want to do this, but do not particularly want to keep an open issue for it at this time.
If we want to discuss whether to do it, let's use e-mail.