[RDY] Add tests for os/fs and os/env

This PR should complete tests of os/fs.c and os/env.c.

3268aab changes the indentation of a whole bunch of tests for semantic reasons and is therefore quite big but doesn't acutally change anything else.
note that setup/teardown run before/after each `describe` block, *not* after each test, which is unusual if you are used to JUnit/Nunit. I tend to use before_each/after_each for busted tests, instead.