Add test ensuring UB can't be introduced

[SUPERCILEX]

If you change the return lifetime of next to 'buf you'll get this error:

$ cargo t --workspace --doc --features all-apis
...

failures:

---- src/fs/raw_dir.rs - fs::raw_dir::_doctest (line 242) stdout ----
Test compiled successfully, but it's marked `compile_fail`.

failures:
    src/fs/raw_dir.rs - fs::raw_dir::_doctest (line 242)

test result: FAILED. 30 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.95s

[sunfishcode]

I must have gotten the versions mixed up earlier., as this does indeed fail to compile. Thanks for all your help here!

[SUPERCILEX]

Sweet, thx!