OK, I'd been meaning to get back to refactoring those tests anyway, so assigning this to myself.

I'm thinking that the right way to go will be to give the test case a more explicit model of "expected platform behaviour" (initialised in setupModule), rather than having that be implicit in a bunch of conditionals scattered throughout the individual test cases.

Then we'd have at least the following cases:

- default is C, POSIX is an alias for C (most Linux distros)
- default is C, POSIX is a separate locale (*BSD)
- default is C.UTF-8 (Cygwin, potentially Android depending on exactly how we resolve that)