Put os::change_dir_locked somewhere else

[brson]

This is a test helper specifically for working around races when changing directories in test cases. As a public API it is very weird and deadlock-prone.

Not sure the best place for it. Perhaps std::unstable, though there's also a std::rt::test module for runtime test helpers.

[catamorphism]

change_dir_locked seems to be no longer. Closing.