Skip test_list_root on OS X (#3772)

[tustvold]

Which issue does this PR close?

Closes #3772

Rationale for this change

I think this fixes the issue in #3772, but as I can't actually reproduce the issue I'm not sure 😅

@alamb perhaps you might be able to see if this placates whatever OS X nonsense is occurring.

What changes are included in this PR?

Are there any user-facing changes?

[alamb]

Sadly it still fails like the following for me

cargo test -p object_store

---- local::tests::test_list_root stdout ----
thread 'local::tests::test_list_root' panicked at 'called `Result::unwrap()` on an `Err` value: Generic { store: "LocalFileSystem", source: UnableToWalkDir { source: Error { depth: 1, inner: Io { path: None, err: Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" } } } } }', src/local.rs:1141:44
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    local::tests::test_list_root

I pushed a commit that made it works for me