Where reasonable, convert tests to free functions

[gadomski]

We switched to pytest in #939, which means we don't have to use unittest.TestCase anymore; we can switch to free functions. In most cases, converting tests should be as simple as changing:

class MyTestCase(unittest.TestCase):
    def test_foo(self) -> None:
        self.assertTrue(True)

to

def test_foo() -> None:
    assert True

While this is a little bike-sheddy, this does have some benefits:

• Enable fixtures, parameterization, and custom hooks
• Reduces confusion for new contributors, who might be unsure of which test format to use
• Reduces the indentation level (a little thing, I know, but IMO it reads cleaner to have less leading whitespace)

[teks]

This seems straightforward so I'll make an attempt. 28 files have import unittest so I may split it into a few PRs so they don't get too long.

However, please let me know if there's a body of tests I should leave alone until some other work is done.

[gadomski]

Nope, go to town 😄