* Repeating the same test is just un-necessary, bad past-Ath.
* Use `raise AssertionError(...)`.
I'd move some of this stuff into functions, but it wouldn't actually
aid readability.
* We've not been building for macOS since the change of maintainership in
2020, so removing that code is long overdue.
However we're leaving in the test for `win32` so as to be explicit that
this should only (currently) be run on that platform.
* Add section comments to better document what each code section is meant
to achieve.
